I found the book fairly well written, and the premise plausable. In a sentence, the author draws on decades of reasearch connecting blood type (i.e. A, B, O, AB) with the relative benefit/harm of specific foods. When I asked my (MD) physician to comment on this theory, he said: "complete nonsense". He explained that there is no widely accepted medical research which agrees with the research cited by this author. On the other hand, a good friend of mine has followed the diet and has had very impressive results. I am starting to follow the diet, and so far have found that I have better digestion. I would recommend the book, but as to its validity, read it and draw your own conclusions.