1) It is very comprehensive and includes almost all the major existing principles in the areas mentioned (scattered in several other works). Here, you have everything.
2) it is extremely practical: each principle is highly developed and supported by very concrete practical applications.
3) much less "New Age" that many books on the same subject.
A point to mention: a few times, the reader must exceed certain formulations or examples which are more suited to our times. This is because the author wrote this book late 50s.
I highly recommend this book, a mainstay in the genre "Pass / Personal Development."