This is an introduction to the use of magic and other occult techniques to aid in personal spiritual development, written by a leading expert in the field in the early 20th century. It was not actually written as a book by her, but is a collated collection of articles she wrote, published nearly 80 years after her death.
There are many typos unfortunately. Mostly the lack of required full stops but also some words that are so completely misspelled it is not clear what they are meant to be. This is not, of course, down to Dion Fortune but to the person who curated these articles and self-published them on Amazon.
Included in the collection is an article Dion wrote on egregores, which Dion here calls group minds. This gives an interesting take on mob mind but also guidance on how this can be used positively. There is also a very thoughtful article on the ways in which sex magic can be used, although the way it is written means it is not particularly obvious that is what it is about.
The magic touched on in this short book is the Western Tradition based on elements of the Golden Dawn as outlined by MacGregor Mathers and Israel Regardie, and as modified by Dion Fortune in her Fraternity of Inner Light. Dion Fortune takes the view that someone studying magic should be an initiate of a group such as (although not limited to) her own, and therefore only gives hints rather than detailed instructions of how to apply the knowledge she imparts.
The final chapter, a Glossary of Occult Terms, is difficult to follow unless you already have a reasonable understanding of basic Qabbalistic concepts. If you don't have this understanding it might be best to skip this.
If you intend to read this in order to find out how to use and develop magic skills, don't bother. But if your intention is to allow magical concepts to penetrate your unconscious mind to help you as you use other methods to develop your skills then it may be worth reading.