ScriptMafia.org put together a list of 21 Design books. Here are the Top 10.
1. Great Web Typography by Wendy Peck
An ebook on Design For Web Developers
3. Don’t Make Me Think: A Common Sense Approach To Web Usability by Steve Krug
5. Letterhead & Logo Design Vol 04 by Rockport Publishers
6. Graphic Design for the 21st Century pdf by Taschen
10. Designing With Web Standards by Jeffrey Zeldman