A great book that is not written for people with digestive problems, but, nonetheless has a wealth of information on how our digestion works and how it can be improved, is Richard Wrangham's Catching Fire: How Cooking Made Us Human.

Wrangham gives great information on why it is important that we eat foods that are easy to digest. If you combine his information with Pimentel's idea that badly digested foods lead to candida and IBS, you get some great ideas on how to improve the diet.