Don't Make Me Think: A Common Sense Approach to Web Usability, by Steve Krug. New Riders Press, 2005.
This thin book provides guidance on the basics of web usability. It focuses on the importance of doing things in a way that meet users' expectations. In addition to some well-thought out examples of re-design, he spends about a third of the book on a simple and cost-effective approach to usability testing. This would be a good choice for your first book on usability.