A Yahoo! Group that I belong to just started a discussion about free books for the Kindle. Besides the places I've pointed out, I found these:
CK-12 Textbooks - AP level texts for Science and Math at Amazon
The organization that is behind the program is CK-12. CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the "FlexBook," CK-12 intends to pioneer the generation and distribution of high quality educational content that will serve both as core text as well as provide an adaptive environment for learning.
Check it out!