Gastro-intestinal physiology reference
If you have no medical background at all, you will need a general text that teaches more than just digestive physiology. That is because you need to understand the function of other systems that have an influence over the digestive system (for example, the peripheral nervous system).

Here are a few good general references to get you started:


Physiology by Linda Costanzo, arguably the most synthetic. Extremely good, but I am unsure of its value for someone who has no medical background.
Vander's Human Physiology, arguably the simplest, used in first year of medical school.
Medical Physiology by  Walter F. Boron, arguably the most complex, I would not recommend it as a first try.


You can start by trying to read the digestive physiology chapters, and look up the knowledge you lack in other chapters.