I found this to be an excellent expansion to the rules for Trail of Cthulhu. As usual Megan did an exemplary job or explaining the contents of the volume, so I'll confine myself to a few additional remarks.
I found the multiplicity of magical explanations and procedures really useful -- especially noteworthy is the idea that all, some or none of them could be the correct explanation/procedure and that "magic" may work for some entirely different reason. The concept of the Magic ability is another useful rule addition and makes the entire idea of magic in the game flow a good deal more smoothly. The examples liberally scattered throughout help to clarify the author's intent very effectively. Finally, the system is simple, effective, and mysterious enough to keep the players guessing even when they are "experts" on the system.
DriveThru did their usual excellent job reproducing this one. Overall, I give it a "5" for its excellence and the quality of the reproduction.
[5 of 5 Stars!]