He is an assistant professor of psychiatry at Harvard Medical School, so I will hold off on calling him a quack, but definitely approaching this with a healthy amount of skepticism.
Although keto & low carbohydrates is an important topic, there are many other causes & solutions discussed in the book. For example: 1- Genetics & Epigenetics 2- Chemical Imbalances, Neurotransmitters & medications 3- Inflammation 4- Sleep, light & Circadian Rythms 5- Food, Fasting & your gut 6- Drugs & Alcohol 7- Physical activity 8- Love, Adversity & purpose in life
If I understand correctly, one of the main ideas is that mental disorders are metabolic disorders of the brain. Then in part 3 of the book, Dr Palmer(author) explains the causes & solutions mentioned above.