I stumbled upon a really helpful website this week called A Book in Time. This site indexes TONS of books by historical period.
If you're studying, for example, American History, you can search first by century. Then each century is further divided into 4 to 5 subcategories. Under each subcategory (ex: 1740-1770) is a list of book titles that fit that setting with a short description and a link to find it. The list contains books for K-12 (I can't find a search function that limits the list by book audience) but each title is clearly denoted with suggested grade levels.
Along with the book lists, the site also has links to craft ideas, reading logs, maps, and a host of other extras. The only downside I can find is that most of the books are older titles. Great books, but I'm not seeing some of my favorite books that have been published in the last few years. Still, this site is definitely worth bookmarking and checking out before your next library visit!