$1.99 Books at Book Closeouts. Click here.
Humor books are 80% off until 6/10/09. Click here.
Walmart carries a bunch of books prices between $5 and $6 that would normally cost $15+ at other stores. They have Twilight for $4. They have 50% off of the top 50 best sellers, too.
Amazon/Target has 45% off certain books.
Pre-order CakeWrecks and get it for under $10 at Barnes & Noble.