Digestion

Free to read here, from Project Gutenberg.

The Art of Invigorating and Prolonging Life

William Kitchiner

The physiology of digestion considered with relation to the principles of dietetics

Andrew Combe

What to eat and when

Susanna Cocroft, also an audiobook

Foods; nutrition and digestion

Susanna Cocroft

On Digestive Proteolysis

R. H. Chittenden