Broken Guts by Anthony Aikman, a retired British doctor living in a Lahu village in northern Chiang Rai.
The book is described as a "rough medical guide for travelers in Thailand and Southeast Asia". it is in an unusual format in that it is handwritten, not typed, but is still easy to read - but that's why it's called a "rough" guide.
It is not just about stomach upsets but every type of medical problem you could face wandering around Thailand, from food poisoning to armpit boils, from jellyfish stings to snake bites. There's even a page on dodgy mushrooms.
It's all written with a sense of humor plus entertaining illustrations and can be found throughout Thailand. It could come in very useful next time you have health problems somewhere out in the boonies.