Many of the principles of graphic design will help a UX designer to create more attractive and easily understood interfaces.
The Non-Designer's Design Book
A fun and short book about the four principles that can easily transform any design into something that looks more professional. These simple principles will stick with you for the rest of your career.
A Primer of Visual Literacy
A more academic book than the first. If you want a fuller look at what graphic design is, and what its building blocks are, this is a good place to start.