The consummate book on UI design is Don't Make Me Think. Additionally, The Design of Everyday Things is enlightening though it is not specifically about UI design.
That said, my experience with UI design has been primarily focused on web applications. I have found the work by Dan Cederholm to be the most enlightening in designing excellent user interfaces for the web. One of the most important aspects of web design is ensuring semantically correct accessible sites and Cederholm does an excellent job describing how to do that in Web Standards Solutions which was followed by the also excellent Bulletproof Web Design.
Though the CSS Zen Garden may not be explicitly about UI design it is an outstanding example of what can be accomplished visually through CSS-based design.
Finally, an often overlooked part of UI design is delivering effective error messages. While, Defensive Design for the Web emphasizes web error design much of what is described is applicable to other mediums. Those responsible for the book 37Signals maintain an excellent blog Signal vs. Noise.
