Crowley's writing is witty and incisive. This is a good book to read for its first four chapters in which he explains the first four "limbs" of Yoga. The next four, in which he attempts to explain the remaining stages of Yoga has much reference to his Magick theory. So if you are unfamiliar with Magick (as I am), the material may lose you easily. All in all, it is not worth spending $9.95 for the book if you are looking only for its Yoga content. If you have access to a good library, I would suggest you borrow it and read it first before you decide to you want to own a copy of it.
This is Crowley' second try at writing on Yoga, and he has expanded the information of Raja Yoga in the section entitled "Yoga for Yellowbellies" which the other reviewer wrote was about Magick. Mostly it talks about Mathematical certainties of the proof of the Yogic states wherein contradiction of the faculty of Reason is inevitable. I think that Crowley has wrote much more then those who never studied him have come to realize: his whole philosophy is concentrated into such texts as "Book 4 parts I - IV," and "The Equinox": no other books will do more then lead the seeker to false beliefs: Perseverando is an excellent display of this.