This was a truly amazing book, and not at all what I expected. I thought it was going to be a clearly laid out explanation of the powers of the mystical Deryni, along with an attempt to explain away the (admittedly few) inconsistencies found in the series. That is this book.
Instead, this book is mainly a philosophical discussion on the basis for the gifts/powers, the ethical considerations involved in their use, and how these relate to religion. It provides possible reasoning behind the reasons why the Deryni were determined to be anathema by the religious leaders of Gwynedd (while at the same time not justifying that reaction). It cites examples from the books in the series to support the points it makes about the use of the gifts/powers. I was particularly impressed by the tone of the book - it's almost like this book was written first, serving as a foundation of the Deryni, rather than almost last, explaining what has come before. The level of consistency, while not perfect, is quite high - high enough to make it simple to believe that this is a real universe and not a fictional one because of the ease with which these things are explained.
If you like the Deryni and enjoy philosophical discussions, you should definitely read this book.