There is an awesome eBook of Balsamiq Interviews by Bob Walsh that includes interviews with nine different Balsamiq users. Each interviewee contributes an interesting perspective on using Balsamiq and its advantages and limitations.
There is also a book about Balsamiq Mockups 2 (our old version) by Scott Faranello. It's called "Balsamiq Wireframes Quickstart Guide". You can get it on Amazon here.
We think this book nicely complements our support website, with easy-to-follow and quick-to-digest steps that will take you from zero to mastering our little tool in a very short time.