The Introduction lays out the intention of this work: to provide an optional systematic approach to magic that can be used within a Trail of Cthulhu game by the characters and/or their antagonists as the Keeper so chooses.

The first chapter - Which Magic? - goes into greater detail about sources. Naturally, Mythos literature looms large, but Lovecraft was by and large unclear about the underlying mechanics of his magic, using it to create the desired effect without much regard to what was going on. Certainly if characters are to have access to magic they'll need to understand the cause and effect of their spellcasting, and so will the more logical Keepers wishing to run NPC adepts effectively. And yet a lot of the fun is in the not knowing... inconsistency may be the hobgoblin of petty minds, but it's central to making your alternate reality really weird. Even in the real world, some people deny the existance of magic completely, while of those who believe that it is real, there are as many theories as there are mages and scholars presenting them! A range of theories are presented as to how magic might 'work' in your game world: pick any or all of them as you please... and even the ones you decide are false may be believed by the protagonists who study or practice magic, of course. Yes, their spells may work, but not due to the theory that they expound!

Next comes The Magic Ability, a proposed game mechanic to codify spellcasting within Trail of Cthulhu. The neat thing is that it does not depend on any of the previously-mentioned theories being 'right' - if you have it, you can cast spells whatever you think is going on when you do so. It can also be used to understand what someone else is doing. You cannot start the game with it, you must acquire it through play - suggestions include the traditional reading of tomes, contact with Mythos creatures or asking an established practioner. Or the scary thought of having to ask Yog-Sothoth for the 'gift' of magic! Once you've awakened the potential, you can use build points to increase the ability provided you have the appropriate resources like a occult library or a Mythos tome, or visit suitable locations (apparently the chamber under the Great Pyramid is worth a couple of points, can't say I noticed when there!). When casting a spell, you can spend Magic points rather than Stability ones although you still need to make any required Stability tests. The chapter ends with an allocation of the Magic Ability to a range of Mythos monsters.

This is followed by Cast A Deadly Spell, which presents a collection of spells characters can learn (or might be on the receiving end of, of course!). True to the style of this game and its literary heritage, the focus is on what the spell does and how it appears to a bystander than on mechanical details. There's also a list of legendary Mythos sorcerers whose names you can throw around to add a touch of verisimiltude. To add a wierd twist, there are ideas for changing things even during the course of a campaign, and certainly to guard against players who think they know what's in the rulebook! Remember too that Mythos spellcasting leaves traces, tracks that the knowledgeable can read and even the ignorant may notice something's awry!

The next chapter, Idiosyncratic Magic Expanded, looks at the rules from the Trail of Cthulhu core rulebook. It's all flavour and wierdness, superstition to some and vital preparation for others. Examples are given for each skill you might care to practise it on. Finally, Magic In Theory and Lovecraft explores some ideas of magical theory based on Lovecraft's writings, delightful concepts for the more intense in-character philosophical discussions, something to play with in quiet times...

The whole work is a fascinating delve through ideas, making the concept of 'magic' work in an appropriate way within within the world of Trail of Cthulhu. The rules for a Magic Ability work well and are consistent with the rest of the system, but the real joy are the underlying ideas that make it all come to life.