Should be the ones that you really think good!
Don't Make Me Think by Steve Krug
Bit more general than the scope of this question, but I think it still applies.
I agree with the guy above for Don't Make Me Think -- I thought a link to a sample chapter of the book might be valuable as well.
I really liked Designing Web Navigation. Although it deals with just one aspect of web UI, that's often the most important part. Users who can't find their way around your site will quickly leave.
Another excellent choice is Designing Web Interfaces, which just recently came out. I'm still working my way through it, but it's chock full of great advice.
It's pretty hard to write a book that's constantly changing. It'd be outdated before it was done being written or published, let alone sold and read.
Instead you should really just be looking for good design blogs, as they will be much more current than any book.
Here are books that are very highly regarded. I have read some, so agree with other by association.
Subject To Change: Creating Great Products & Services for an Uncertain World by Peter Merholz
Communicating Design: Developing Web Site Documentation for Design and Planning by Dan Brown
Contextual Design: A Customer-Centered Approach to Systems Designs (Interactive Technologies) by Hugh Beyer
Designing Web Navigation: Optimizing the User Experience by James Kalbach and Aaron Gustafson
Designing for Interaction: Creating Smart Applications and Clever Devices (Voices That Matter) by Dan Saffer
Designing for the Social Web (Voices That Matter) by Joshua Porter
Designing Interfaces by Jenifer Tidwell
Designing the Obvious: A Common Sense Approach to Web Application Design by Robert Hoekman Jr.
Designing Web Interfaces By Bill Scott
The Elements of User Experience: User-Centered Design for the Web (Voices That Matter) by Jesse James Garrett
Information Architecture for the World Wide Web: Designing Large-Scale Web Sites by Louis Rosenfeld and Peter Morville
A Project Guide to UX Design: For user experience designers in the field or in the making by Russ Unger and Carolyn Chandler
Don’t Make Me Think: A Common Sense Approach to Web Usability, 2nd Edition by Steve Krug
Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics (Interactive Technologies) by Thomas Tullis
Sketching User Experiences: Getting the Design Right and the Right Design by Bill Buxton
Mental Models: Aligning Design Strategy with Human Behavior by Indi Young
Landing Page Optimization: The Definitive Guide to Testing and Tuning for Conversions by Tim Ash
Web Form Design: Filling in the Blanks by Luke Wroblewski
The Back of the Napkin: Solving Problems and Selling Ideas with Pictures by Dan Roam
Observing the User Experience: A Practitioner’s Guide to User Research By Mike Kuniavsky
Courtesy of UsabilityNews.