As I get time, I want to put on my librarian hat and highlight some commonly available resources that are invaluable for writers. You savvy researchers are probably familiar with some or most of them, but I hope I have a trick or two up my sleeve that will surprise you.
Some of these resources are just neat websites that don't require a login. Others are quite expensive databases you can access for free via your library's website, and may require a library card number for access. Some libraries will let you apply for a card online which lets you access databases but not check out books--this is very helpful if you live far from your library or don't have a chance to make it in.
Are you a non-student who longs for access to your local college or university's library? Ask if you can get a card! Sometimes they are in a cooperative program with the public library system or are otherwise willing to give cards to local non-students.
Don't forget that you have access to a much wider pool of physical resources than you thought you might. Many libraries offer interlibrary loan services so you could request a book from halfway across the country and have it loaned to you for free or cheap. It's also worth asking around to see what kind of special collections or ephemera your local public and academic libraries might have. For example, my public library system has a fantastic Audubon collection and my university library has a collection of postcards and correspondence from Abraham Lincoln. Librarians love to talk about special collections! Go in and ask and you never know what treasure trove you may unearth.
Ask a librarian for help doing advanced searches in the catalog computer. They're usually a little more picky and complex than an online bookstore's search function, but can yield great results if you know what you're doing. They're all different--for example, my catalog system uses a $ as a wildcard and not the standard *. You can do a lot with boolean search functions too, using AND, OR and NOT. (For example, potter NOT harry would eliminate J. K. Rowling's books from your results.) This is so useful if you want to search for books with specific plot elements--you can learn how to limit to just children's book results, or only books published from 2000 on, or whatnot.
When in doubt, ask a librarian! If she is grumpy, ask another librarian! The stereotypical librarian dream job is helping writers do research, so believe me, you will find somebody who adores helping you. Don't give up if the first librarian you talk to is not that person.