A lot of my site is about UI design. There are several hundred links for resources.
Unlike graphic design, it's not about what your UI mockup looks like in PhotoShop. You're going to have to learn about user testing methodology,
design personas and
wireframe design.
In other words, it's not a style to fluff up your portfolio with. UI design is a completely different way of thinking. Did the user complete the task -- as in we tested this and witnessed the user doing it.
Books:
Psychology of Everyday Things
Tog on Interface
About Face, and The Inmates are Running the Asylum by Alan Cooper
Web:
Jacob Nielsen's Alertbox. Start with
Growing a Business Website: Fix the basics first
First Rule of Usability? Don't Listen to Users
AstTog
A Quiz Designed to Give You Fitts
Denial in Web Design: Elephants in the Living Room
A List Apart
Topics: Design: User Interface Design