I'm learning how to do some belly exercises to help with my back pain. While lying on my back with my knees bent I put a heavy book on my stomach. Then I'm supposed to pull my stomach muscles in. How does the book help?

The weight of the book gives an outside cue to "wake up" the abdominal muscles. They pay a little more attention when there's an added stimulus. The book weight combined with the pull of gravity in this position work together to improve your muscle contraction.

Sometimes people start with a fairly heavy book. Over time the book is traded for a lighter one while still contracting the muscles with the same vigor. The goal is to be able to contract the abdominal muscles fully on your own without the added help.