These are a few of the books that I find myself recommending again and again.

Foundations of User Experience Design
If you're interested in learning about HCI, these books are the place to start -- most are required reading for any experience design program, and for good reason. They're easy to read and jam packed with great advice on how to approach design.

Information Visualization
These two are more domain specific, and are interesting if you're designing lots of data visualizations, e.g. charts and graphs.