There are a lot of books out there, I have at least twenty-three that I can count on my shelfs just sitting here without going to look for those that may be misplaced. There are good and back things about all of them. The question you need to ask is how much knowledge do you have and what kind of trees do you plan on growing? Are you interested in traditional bonsai with temperate trees or indoor bonsai with tropicals?
In general the best and easiest book to understand is the Sunset book, I don't now remember the official title but it has held up for many years. For someone just starting out it is understandable and not particularly complicated or expensive.