1) the country. The Middle Ages in France and England are notoriously different, as are Los Angeles and New York in "the Thirties." Face it: the USA is several times the size of Europe and, except for the use of English, can be very disparate. Its regions count like European countries. Make this as tight as possible: Yorkshire, not England, or even just York and its vicinity is your story isn't travelling much. Otherwise, you'll get a bunch of irrelevant stuff about London.

2) the class, age, and gender of your main character. A 25-year-old male farmer lives a different life than a wealthy merchant's 16-year-old daughter.

3) a year, not a decade or a century or era. It is better to say "around 1923" than "the Twenties," starting with "the Twenties" is not an integral period, except on a calendar, promoted by really lazy history writers (I won't even call them historians) or editors. Almost all decades are fake divisions. 1928 is rather different than 1923, and 1932 very different than 1938, but 1928 and 1932 aren't as different, though they are in different calendar decades, and on different sides of The Crash. In the 14th century, 1328 is way different than 1378, even in France. (There's a little pan-Eurasian trauma in there called the Black Death.)

4) what you want your character to do, that's important to you and the story. What the character does is called the plot, and if your plot requires going to the Holy Land on Crusade, that's a certain time window because those Crusades weren't happening all the time. If the character is going to be an early woman doctor or lawyer, there's a cut-off start date for different areas of the world. If the character is going to be a wife who owns property, it has to be after the Married Women's Property Act and such. Your plot may fit another country or era that you couldn't well pick, with your vague sketchy knowledge. Tell us what you need your character to do, and we can make better suggestions. No, no one is going to "steal your plot" from this. Starting with there are no such things as new plots: it's all been done before. You're stealing someone else's already.

 1). A general history of the time, not over 200 pages.
I would count crawling links all over Wikipedia your first day. Do remember wikis can have unproven statements, but usually in contemporary controversial subjects. But so can printed encyclopedias! I'll never forget a 1990s Britannica that claimed the ancient Egyptians had iron wheels on their chariots, when no one ever used metal wheels on chariots. Remember, opinions on some interpretations change, though the facts like dates rarely do. Use Wikipedia's Create a Book function to save all the good pages, then organize it and download it as a PDF on your machine for fast reference.

2.) An "everyday life" book of the period.
You don't have time to flail around with all ages. Most of these cover "the Middle Ages" or "the Victorian Age" and that's quite loose enough -- really, too loose. Look in the YA section of the library, not the children's, and check adult, too. If it's Near History, you can also crawl Retronaut, for a couple of hours, which has more humor than not. They do have a chunk of 1800s and earlier, mostly odd objects like artificial hands of the 1500s.

3.) General transportation
Find out how people get around town, then worry about long distance only if someone has to sail the Channel or fly the Atlantic or take a train overnight. There are sources in the 50 Books guides.

4.) General costume
12.) Specific dress styles, for your decade.
   You need about 20 minutes on general costume, just to see if your guessed-at year is somethng you can't deal with. Costumers Manifesto is a free online resource for this. Wikipedia is poor on costume.
   When you have a year, you want the main outline of clothes, so you know if ladies have lots of petticoats or one of those hardware underskirts like hoop skirts, bustles, or panniers. Search for your year and place: again, French and English fashions can be rather different, let alone Norman and Anglo-Saxon.
   In Near History, dress changes fast: anything five years old is out of fashion and next year's is only barely imaginable. If you're setting in 1976, you don't care what they wore in 1969 or 1978. (And please remember, by 1970 "Mod" ="out of fashion": I'm hearing people call Disco shag carpet Mod.)
   Also free: A History of Costume by Carl Köhler, edited and augmented by Emma von Sichart, and translated by Alexander K. Dallas (1930; New York, G. Howard Watt).
   That gets you into Near History. Then you look for copies of Photoplay Magazine and other fashion sources at Internet Archive. You can get a general idea of your specific costume just in an hour or two, just so you know where the hems are and whether men wear trousers or tights. Don't get married to any descriptions of "sexy clothes" you come up with: you probably have them wrong for the era. Just insert <<describe how sexy he/she is>> and go on. I still remember, like a vivid nightmare, a romance where the heroine wound up dressed from the latest Frederick's of Hollywood catalog though it was something like 1830 and there were no lace stockings, suspender garter belts, or the like, not to mention another that had a Cavalier wearing a black satin shirt and skin-tight breeches (white linen, always, and really baggy breeches). Descriptions make words, but they don't make story -- if you want to finish the draft with the important stuff, skip the sunsets and ballgowns and get on with the interactions.

5.) Specific transportation The later in time you are, the more this matters, as you have more choices. For now, just say the person drives a carriage, and worry later whether it's a curricle or a dogcart. Especially December for which colour, year, and model of sedan or roadster. Me, I just ask my husband for Near History automobiles: he carries it in his head. Ask on the forum and get back to writing without waiting for the answer.

6.) Etiquette, and I don't mean morals for maidens.
I still consider this crucial because these are the books about how the upper classes spend their days, and what the middle classes imitate. You can learn so much about the world through these. When I was in high school, my best friend got a 1914 etiquette book. The world it revealed, of calling cards and paying visits, precise parameters for dress by time of day, event, and season, was a stunning revelation. I've been collecting old etiquette books ever since as one of the best portals to the past. However, before 1830 or so, it's Lord Chesterfield from the 1780s; from there back to the Renaissance, it's Castiglione, then The Babees' Book, for English Renaissance, then for the Middle Ages it's the Knight of La Tour Landry. In earlier periods, you can learn as much if not more about etiquette reading the fiction of the day, like Chaucer's Canterbury Tales. Check the Library of Congress's American Memory site for their Dance Manuals, of all things. It isn't all American, in fact none is until the early 1800s, and foreign books continue strong after that.

7.) Spectator entertainments, a general overview.
8.) Self-entertainments
Let's lump these together. Don't devote 2000 words to a theatrical entertainment's description if you haven't studied theatre of the time: put down <<description of play>> and pass on to what's discussed in the box or on the benches. It was all way different before electric lighting. Equally, don't put too much wordage into the description of card play when you may find out they didn't have cards yet, or only the wealthy did. For example, Texas Hold 'Em is a relatively new form of poker and would look way out of place in the Disco Era, let alone earlier. If your story is set in the theatre, you do need to know the styles and limits of their time. My suggestion is to go to Amazon or Alibris and get a textbook for Theatre History. Also, don't rely on everything before 1960 being "ballroom dancing" position: your cavalier is not closely holding the lady he dances with, but only touching her hand.

9.) Public Hospitality, including food and dining, including what sort of public dining was available, as well as where a stranger in town stays.
Restaurants are modern. This topic includes table manners, as in some times and places men and women do not dine at the same table, or the rigors of seating by rank mean you don't get to choose who you sit with. An etiquette book may cover it, or in earlier times you will need a book or site on dining out or public hospitality. It's naturally important to know whether your characters will go to a hotel, a hummums, an inn, a caravanserai run by the government, or put up in the back rooms of the big temple.

10.) Recipes for period food. Just type <<describe the sumptuous/vulgar/piteous meal served>> and go on. December for what that entails.

11.) Marriage and family.
Modern marriage laws and attitudes are generally more or less wrong before 1960. The farther back you get, the less married women are human. Kitchin, S. B. , B.A., LL.B.; A History of Divorce ; 1912; London : Chapman & Hall, Ltd., covers all but Near History. Frankly, unless you watch 1930s movies, you've probably never heard of a breach of promise suit, but I pivoted my 2012 NaNo on it.

13.) Religion for the time and place. Only if you're going in religious places. If your plot is very oriented to religious subjects, you may have this down well enough from whatever got you interested. But if you haven't done your reading, it's not wise to pivot your plot on an uneducated guess. 14.) A fat history book of the area and century as an introduction 15.) A history of the most influential country at the time (country A). 16.) A history of its rival (country B). 17.) A biography of the leader of country A 18.) A biography of the leader of country B 19.) A history of the country you are setting in, general.

20.) A history of the country you are setting in, that era.
A large encyclopedia article may do. You don't want to lose a war or misplace a Crusade.

21.) A biography of the leader of the country of your setting.

22.) An everyday life for the commoner/lower classes of your time and place.
This is both the peasant farmer and the outlaw.

23.) An everyday life for the upper classes of your time and place.

24.) An everyday life for the middle class of your time and place.

25.) An everyday life for women of your time and place.
Because the other books will often ignore how women spend their day, or give an inaccurate stereotype.

Once you know your city or country, and year, these four are the most important books you can get. This is life, and how your characters think and believe, and what shocks or soothes them, what they fight for or against. That is, if they're good everyday life books. An "Everyday Life in the Middle Ages," claiming to cover life for 1000 years, from the Arctic Circle to the Mediterranean, and from the Atlantic to the Danube, is obviously giving you very sketchy information, with bits from all over or having the practices of one time and place stand for all -- which is just wrong. Especially, England rarely represents common Continental practices, whether in 50 BC or 1860.

26.) An auto/biography of someone like your protagonist, or a book as much as possible focused on people like that. Mainly because these are so hard to run down in some cases. If you can find one, or are inspired by one, it often gives you a relevant chunk of the everyday life.

27.) A book on houses and furnishings of the period, if possible.
Because you may make the house or furniture integral to the plot. You can't hide in the closet when the houses don't have closets, or sneak down the hall past closed doors when there aren't halls, just rooms opening into rooms. What's the heating? How much privacy do you have? Illustrated History of Furniture from the Earliest to the Present Time by Frederick Litchfield (1903; London: Truslove & Hanson Limited; New York; illustrated by John Lane 1892-1903) will get you through some of the basics for free. In Near History, you can find pre-fab house catalogs and decorating books at Internet Archive. In the later 1800s Godey's Lady's Book had a house plan every month.

28.) A book about courting, romance, and sex of the time.
You need it if you can find it. If you can't find it, fake it, though that's asking a lot of faking in a romance novel. Which may explain why many histfi writers say histrom is only fancy-dress romance, not usually historical enough to mention. With occasional exceptions of wonderul care that let me immerse myself -- yeah, most histrom ranges from the cliche to the hideously ignorant and confused, unless it's Western histrom. Harlequin Historicals, I'm looking at you.

29.) A book for naming historical characters properly. Baby name books are so wrong. They are all modern names, including that what is male or female may have changed. But for November you can use anything! Just don't get married to it. Ashley may have to become Helena or Maude to not sound stupidly modern. There may be sites for you: check the 50 Books guides for your period.

30.) Medicine of the time and place.
For general use, try the freebie, A History of Medicine, 1945 Thomas Nelson & Sons Ltd;. London, Edinburgh, Paris, Melbourne, New York; it covers around 500 BC to 1940 CE. There's a nice history of the treatment of bone fractures up to the 1500s from 1936: it includes information on the Winnebago and Dakota tribes, along with the expected European.

31.) Climate, weather, and seasons.
This may be one book on climate history that you use for many periods. The Little Ice Age: How Climate Made History, 1300-1850 by Brian M. Fagan (2001, Basic Books) is an example, as it covers what are often considered the most popular centuries for histfi, 900-1900.

Of course, you need to go to the Perry-Castañeda Library Map Collection site for historical maps and other map sites.