The best book I've ever read on Usability/Interaction Design, and one of the best books I've read period, is a book called About Face 3: The Essentials of Interaction Design by Alan Cooper.
It's a fantastic book because it talks about a lot of fundamental concepts behind interface design for any type of interface, not just on the web. Understanding these concepts will help you make better creative decisions, especially when designing something that hasn't been design yet (like a new product or type of social website), not just help you copy what's already been done.