This is the star psychiatrist psychoanalyst of now. That is not to say that the book is bad, but it clearly needs to conform to some fads in order to be a best-seller.
His main idea is clever and, ad hoc, obvious (generally, it's a hint that it is probably a good idea): emotions have a physiological basis, their repression also does. Not expressing emotions generates physiological stress (high levels of cortisol), and doing so chronically creates chronic stress and, in some cases, diseases.
He goes through several of them (ALS, MS, rheumatoid arthritis, fatigue syndrome, IBS, endometriosis, cancers...) and for each, unearths old epidemiological studies that also search for psychological and social variables: these diseases and their negative prognoses are correlated with bad life experiences, especially childhood experiences (specifically: ACEs, adverse childhood experiences), which shaped the way people react and deal with their emotions.