Specifically, the examination of the Buddhist Realms of existence (of which there are 6 I believe) and its relation to our states of mind. In doing so, we are treated to stories of the personal struggles of his patients as well as their parallels to Buddhist concepts (most notably the 6 realms).
I would recommend this book to anyone looking for a good INTRODUCTION (as the depth could be expanded further of course)to Buddhist philosophy within our current scientific concepts, as well as psychologists looking to expand their world view and see ancient Buddhists as their predecessors. A well written and caring book.