Jenni, a good way to get books is make a wishlist on Amazon, and whenever it's time for a pressie, get someone to buy it for you. I cheat - I write reviews for a shipping site and they pay me in Amazon vouchers! I have about £40 so far which I'm trying to make £100, then I can buy some of the books in my wishlist. We also have Bookmooch, which I think is international. Basically, you gift a book to someone who is requesting it, then you earn a point, and you can use that point to ask for a book you want.
I have hundreds of books, so I've had to stop myself buying more - although I couldn't resit the Vegan Cook's Bible which I bought in the store yesterday! What a book! x
Editted to add: Why does cheat appear twice? I tried to edit and it's only there once - I've noticed that on loads of other peoples posts too! Rabid_Child = it's on yours with the word store!