I've been helping set up for a big, church yard sale we call a Give-and-Get, and I've been enjoying the treasury of books flowing in. Parents may balance the budget by cutting out new books for themselves, but they'll stretch the budget to provide something educational for their families, and as a result, some wonderful, slightly-used, non-fiction books are available for cheap or free.
I picked up a great book on castles last night. I don't write medieval romance, but you never know when knowing the ins and outs of castle construction may come in handy. I might have a character travel to Europe and be held captive in a castle somewhere. Lots of possibilities. Best of all, each time I learn something new, fascinating new ideas and story lines fill my brain. It's awesome!
Great research can be conducted online, and at libraries, of course, but when you're stuck, when you need an idea to break the mental log jam, sometimes thumbing through a few books will present a myriad of never-before-considered ideas.
So make a list of topics about which you could use a few good research books. I keep a few on military weapons, lots of historical reference books, several on colonial life, emerging religions in America, and medical books that detail treatments available during certain periods. I'd like to get my fingers on some books that illustrate historical fashions. It would save me countless research hours to have that info at my disposal.
So make your list, and hit the yard sales. Amazon is also a good choice for used books, but be frugal. Consider how many of your published books you'll have to sell to recoup your research investment. I say, go yard-selling first.
Here's a delightful video based on Macauley's "Castle." Are you inspired now?