Moon Magic is a follow on from Dion's earlier novel The Sea Priestess. Part of the book was written by colleagues after Dion's death but that does not make the book any less spectacular.
The story continues with Morgan le Fay the main character, who's name in this story has changed to Lilith le fay. In the story she is living in London and on the lookout for a new "priest" to help conduct her magical experiments. This book is more than just a story, it can also be a manual of magical practice if read and understood carefully. Dion Fortune was an incredibly fascinating woman, there is not a great deal known about her as she liked her privacy, or perhaps she liked the air of mystery that that she had. To read her books is to get to know her better, and Moon Magic shows she was a true priestess with a lot of inner strength and power.
If you liked The Sea Priestess, you won't be disappointed with this book :)