True20 Adept's Handbook -- I really like True20: with that said, however, I am not really happy with T20AH. Although it IS a thorough examination of the arcane, the profane, and the occult, my games tend to treat magic as "mysterious" and "intriguing" -- not macabre and gothic. You will find much information pertaining to the darker side of magic: witchcraft, rituals, necromancy, sorcery, talking to the dead, etc.*
I bought it to supplement my super-hero and Star Wars (TM) games that I run with True20, but it did very little to help me with those games.
However, I rate things based on value, so I am giving it a 4 / 5 stars, the rating that it deserves for being a detailed and entertaining booklet.
If you like to run authentic magical campaigns, then this book is for you -- if you want to run comic heroes, stick with True20 rules or M&M.