The Fasting Cure is both a fascinating and slightly dangerous book. It was written by Upton Sinclair, a respectable and accomplished journalist, but no doctor. In this case Sinclair seems to have been taken in by his subject matter without using quite enough of his critical faculties.
Fasting has been a part of health and religious practices since time immemorial, it can be healthy in moderation and has various well-documented effects on the body. From this perspective Sinclair's book is worthwhile for the hints and tips towards more sensible fasters and also as a historical document of the "fasting fever" which occurred around the time of writing. One thing fasting is not however is a cure for cancer, or for several others of the diseases claimed within. Another problem with this book is the claim that fasts for up to 40 or 50 days are common and healthy, modern medical advice would suggest that this is far too extreme and very dangerous.
Rather than a single book this is a collection of articles written by Upton Sinclair on the topic of fasting as well as letters on the topic which he received. His remarks and comments on these are also included and serve to bind the parts together. Readers may be interested in this work from a historical or anthropological point of view especially, but should not follow the instructions without close contact with their physician.