It is a very good book with rock solid developmental exercises. However, it is Golden Dawn derived, so it’s method of evocation includes the LBRP, the Hexagram ritual, and the vibration of Hebrew godnames, as well as the Spheres of the Tree of Life.
It was one of the first books I have ever read that gives exercise sets for developing the ability to see and hear spirits, and they work.
The book is worth it, in my opinion, for the exercises alone, but the list of 50 spirits and their sigils is also pretty useful. It is the book EA got the spirit Mesphital from, that he included in Works of Darkness, as well as his evocation course.