Heya! I hope you don't mind if I copy my remarks from an earlier post. (That person was interested in WW2, so just substitute accordingly.)

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"Read a bunch of academic books," while dead-on accurate for above average knowledge+specialization, isn't very useful as a starting point--mostly because academic books are purposefully geared for someone waaaay past the starting point. It's tough because you wind up in a place where it really seems like "you have to know something before you can learn it."

My entire academic career has been me falling off a cliff into new areas of study, so, naturally I have plenty ideas of how to do this RIGHT instead of wrong like I do. :D

First--yes, I think it's a good idea to start by reading, say, the Wikipedia article for the broad broad topic. Now, the article is going to be (a) dry, maybe boring, (b) probably long, (c) a mix of intricate details and stuff you want to skim because you already know.

The key is--don't get bogged down. Don't take notes; don't try to memorize anything. You aren't trying to learn details right now.

You want the General Framework--a timeline, the major players (e.g. which countries are on which sides in WW2, not, like, a full list of colonial American leaders and their biographical details), the most basic broad contours of the situation. You are going to want to know that Germany starts the thing by invading Poland, although Japan is already at war with China; the U.S. gets involved at Pearl Harbor; meanwhile, Germany is slaughtering Europe's Jewish and Romani populations.

So essentially, you just want to acquire the scaffolding or outlines that you are going to slot all the stuff you actually learn into. (I picture in my head kind of a topical Tic-Tac-Toe board. First you "draw" the lines, then as you read you'll fill in the marks.)

If you can find a good pop/academic crossover history on your topic, that's a good place to go next. The AskHistorians booklist will usually say, in the descriptions of the book, what's a good place to start. (Mann's 1491 for the European imperialism era, for example.) This will also probably be a reasonably broad overview, not something to stress about memorizing.

...Actually, "don't stress about memorizing" is THE most important lesson here. Don't bother. It will be stress and make things unfun. Don't worry--as long as you keep reading, you will eventually realize you legit know the stuff (recognize the names, places, events, start to make connections the authors don't) on your own. It's...it's a really cool feeling, actually.

Anyway, if you have a decent enough General Framework, you can start reading more specialized stuff to begin to fill it in. Here's where it can get tricky. The AH booklist is good; you can ask for more direct recommendations on sub-topics that interest you (say you really love reading about women war heroes; maybe you're into home front stories; inspired by one book later on, you develop an interest in scientific progress resulting from military innovations in wartime).

My personal favorite strategy is to see if I can find a PhD exams reading list on a topic. "World War II" might be difficult, since that's not very often a focus of PhD research these days, but "20th century military/political history," perhaps, or "European history" or some such. For U.S. Civil War, you can probably remix a list out of stuff on 19th century U.S. history, African-American History, maybe Southern history...etc.

Many of the books will be from academic publishers and very very expensive. If you're in the U.S., even your public library should have access to Interlibrary Loan, through which you can get a short-term checkout of academic books from university libraries (hopefully). Otherwise, if you live near a major uni, you can most likely use the library in-house--meaning physically be in it--as a guest. Or you might have...your own...ways of obtaining books (say, ebooks in particular) without a university connection.

Anyway, if I were going to start a new area of study systematically, that's how I would go about it.

I hope this is helpful--please let me know if there's anything I need to clarify or if you have further questions. :)