If you're interested in Kurtz's Deryni series, this book will help you further explore that world. It gives some additional details into the history of the Deryni and explains much of they magic they use in the many Deryni books. It's also a good way to fill in the gaps. If you want some specifics on the magic used in the novels, this is the book you need. Especially for those trying to figure out all the references to warding cubes in the novels. It's all well explained here.
That said, it's not a good stand-alone book. If you haven't read any Deryni books (or didn't enjoy them), this one is likely to be more than a little boring. It is written rather like a history, so it's a bit dry. You need to be a fan of the many Deryni novels to get anything out of Deryni Magic. Remember that this book is not a novel. It's not a story with character development, or really any characters at all. It's a companion book, a grimoire, and should be approached as that. If you're looking for a story, this is not the book for you.
All in all, I'd give Deryni Magic 4 stars out of 5. Why not a full five? The prose leaves something to be desired. Even a history can be written so that it's not entirely boring.
