This one's easy... Since I am fairly low on the socio-economic ladder at the moment, I get almost all my books from the library.  I found it at The Book Junkie's Bookshelf:

1. Anyone can join. You don't need a blog to participate. Just create a post for the challenge and link to your challenge post in the linky below. --Non-Bloggers: Post your list of books in the comment section of the wrap-up post. 2. There are four levels to this challenge...Pick your poison: --The Mini – Check out and read 30 library books. --"Fun" Size – Check out and read 40 library books. --Jumbo Size – Check out and read 50 library books. --Mega Size – Check out and read 51+ library books. (Aim high. As long as you read 30 by the end of 2011, you are a winner.) 3. Audio, Re-reads, eBooks, YA, Young Reader – basically any book counts just as long as it is checked out from the library. Books MUST be checked out like with a library card, books purchased at a library DO NOT count. 4. No need to list your books in advance. You may select books as you go. Even if you list them now, you can change the list if needed. 5. Crossovers from other reading challenges count. 6. Challenge begins January 1st 2011 and goes thru December 31 2011.

I'm going for Jumbo Size as a goal.  I'll be listing my choice as I go, although I already have one to start the challenge:

• The Lightkeeper's Daughter by Colleen Coble