If you only get one book on interaction design, this is the one.
I picked up the second edition when I was just starting out as an interaction designer; it was a great primer and filled in a lot of the missing pieces for me. Now that I've been at it a while, it's still the book I go to whenever I have a question. I found the book reads well cover to cover, and also serves well as a handbook. The info you need on a topic is usually well contained in a section.
Not only does this book cover the general principles and theory behind interaction design, but also provides lots of real-world practical information. The writers call on designers not simply to follow rigid interaction design rules, but to create elegant, informative and respectful interfaces. That's a loftier goal, and this book give you the tools to attain it. The updated edition also spans new technologies and paradigms that have emerged, and covers them thoroughly.
Cooper has an unrivaled depth of experience to draw on, creating a truly comprehensive book.