What is a good book about soup?
If you check out the "Seasoned Advice Blog", @Yossarian has posted the first two parts of a three part series entitled "Three Books for Every Kitchen". In part-one he describes The New Best Recipe from Cooks Illustrated. I will direct your attention to Chapter 2: Soups. Not only will you find a rich collection of recipes, but invaluable advise on every aspect of soup making.

Part two of the series, "Ratio", also contains several lessons that will improve your soup making skills.

Not only will these two books help you with "Soups" they will enrich your overall kitchen life, in every aspect of cooking.

(note: at this point, part 3 is still pending, but I will bet it will be a winner as well)