The Crimson Spell - F. Goldsborough, Constance M. Burge I enjoyed this book in the same fashion I enjoy the series as a light-hearted guilty pleasure. The writing veers to the slightly cheesy "Fear not!" "The witches must be destroyed!" but then again so does the dialogue and on the show ;)
Fortunately the author also pokes fun of the cheesier aspects of the series with gems such as:

Bad guy: "You may address me as Master."

Piper: "Master of what? Hokey Halloween costumes?"

It's a fun read and very true to the series.