If you are into usability and interface design these are a must; a great book collection by Smashing Magazine. If you are a web designer or developer some will actually help you not make common mistakes, others will take your techniques farther, but understanding them will surely improve the usability of your projects.

Here is their list:

1. About Face 3. The Essentials of Interaction Design by Alan Cooper, Robert Reimann and David Cronin

2. Prioritizing Web Usability by Jakob Nielsen, Hoa Loranger

3. Designing the Obvious. A Common Sense Approach to Web Application Design by Robert Hoekman, Jr.

4. Don’t Make Me Think. A Common Sense Approach to Web Usability by Steve Krug

5. The Design of Sites. Patterns for Creating Winning Web Sites by Douglas Van Duyne, James Landay, Jason Hong.

6. The Design of Everyday Things by Donald A. Norman

7. Designing Interfaces: Patterns for Effective Interaction Design by Jenifer Tidwell

8. Designing for Interaction: Creating Smart Applications and Clever Devices by Dan Saffer

9. Designing Interactions by Bill Moggridge

10. Envisioning Information by Edward R. Tufte

This is only a partial list; for the complete list (they include editor choices) and for more information on each book read the Smashing Magazine article.