With over 450,000 copies sold, this book is arguably the best book of its kind. What's it useful for? Mainly to help the reader (from the weekend athlete to the athletic trainer to the professional bodybuilder) figure out what exercises work what muscles.
It's neatly divided up into sections (arms, shoulders, chest, back, etc.), so all you really have to do is flip to one of these sections and it will have detailed pictures of various exercises and exactly which muscles are involved.
A great reference to keep have around, I give it five stars easy. Readers who lift weights regularly might also be interested Treat Your Own Rotator Cuff
to avoid shoulder problems a lot of lifters eventually get.