Jim Kajiya makes an excellent point about this in How to Get Your SIGGRAPH Paper Rejected, which I believe applies equally to books:
Ivan Sutherland once told me that Scientific American articles are constructed so that you can get the point of the article just by reading the captions to the illustrations. Now, I'm not suggesting that you write a technical comic book; but you should take a look at those SIGGRAPH papers you were initially attracted to and see how they went about getting their point across.Given that a reader is likely to read captions for figures before reading the main text, it is important to write good captions! A trick I use is to write the captions before writing the main text. This has worked pretty well so far. You need to be careful that you don't include too much in the caption but a little redundancy isn't bad: I like captions to reiterate a key point from the main text.