It's hard to go wrong when Lou Rosenfeld and Steve Krug both recommend a UX book, but I was really impressed. I've dabbled in some eye-tracking studies (poorly, I'm realizing) the past couple of years, and this book was an excellent guide to how to do it right. It really takes you through the journey of how to ask the right questions, design an experiment around those questions, and answer those questions with the current state of eye-tracking technology. The book is very accessible and anyone looking to spend ANY money on eye tracking should buy a copy first.