Try looking up Celiac Disease or Celiac Sprue; Chrones Disease; Inflammatory Bowel Disease; and that great make-believe disease now given the name of IBS or Irritable Bowel Syndrome. IBS is a fabulous catch-all scapegoat for lazy doctors who are unwilling to perform the necessary investigations to get to the root of the patient's real problem. Anyone with a tummy ache these days gets diagnosed with IBS, and usually told that fiber will cure them forever. BAH HUMBUG!
Have you tried doing any research on the net? There is an untold wealth of information, but of course it isn't in book form.
Just type your subject into a search engine.
Also - Mayo Clinic
Usually one link leads to another, etc. You just have to put forth a little effort and time. -:)
Good luck with your studies
Tmblwd