JellyNeo's Item Database lets you search by shop (so you'll have to go down and pick all the shops with "books" in the title), and you can sort the results by price -- and their prices are based off the SW, so you know roughly how much you'll pay.
Jelani's right about Keyquest prizes; as each season ends, I try to pick up all the book prizes that my pets haven't yet read.
Here's a list of seasonal prizes with the current prizes at the top, though the Halloween books might still be discounted a bit. Other cheap books can be gotten from fishing, Wheel of Knowledge, and Weltrude's ToyChest.