Cabal is, unsuprisingly, a necromancer. He's traded his soul for the power of life and death, but now he's discovered that his soulless state is impacting his (otherwise rigorous) experimentation. Cabal is exactly the sort of meticulous proto-scientific sort that can't stand this sort of unpredictable involvement in his research. Therefore, he needs his soul back.
Satan, ever up for a good time, is willing to wager. If Cabal can get 100 souls in the next year, the Devil will return the original (slightly worn for wear). Hell, being an accommodating sort of archfiend, Satan will even chuck in the means of soul-gathering: a carnival.
Cabal knows that the Devil will cheat, but any chance is better than none, and the narcissistic necromancer has a very high opinion of his own cunning.
What follows is a somewhat-blackly humorous series of episodic adventures as Cabal and his cronies attempt to outwit the Devil and reach their quota. The "somewhat-blackly" comes from the fact that Cabal, despite name & profession, isn't really a bad guy. For the most part, he's off preying on those beasties and blackguards that are even more reprehensible than he is. Despite some efforts to create moral quandaries, there's never really any tension about it. If Cabal were unlikable, the book wouldn't work. Fortunately, the reader can back him with only the barest amount of unease.
The book is also very funny. Howard has a very polished, supremely composed style. The closest comparison, if one were necessary, would be Stroud's Bartimaeus - except without the tangential footnotes. Cabal's dry sarcasm combines with a strong sense of comedic timing (always tough in written form) to create a book that's slick and wry, as opposed to laugh out loud. There are a few set-piece comedic bits that are perhaps a little over-composed - the occasional stretched, near-Pratchettian silliness, for example - but largely, Howard is channeling a voice of his own.
There is, however, something about humorous genre pieces that necessitates an episodic structure. No one since (or possibly including) Adams has been able to create a holistic storytelling experience and keep the laughs coming. The carnival set-up is a good one: Cabal moves from town to town, having a different encounter in each. But Johannes Cabal is a child of the TV era. Each chapter has a beginning, an end, some appropriate chuckles and a nod towards a larger plot arc. For Howard, it works - down to the special double-episode season finale. (And appropriate cliff-hanger to start season 2).
Johannes Cabal the Necromancer is a clever, well-crafted book. It sets up a sequel (which is already out) that I'm extremely curious to read. I could see the central conceit of the story getting very old, very quickly. Or, in the hands of a talented author (like Howard seems to be) becoming a cult cultural icon. Either way, this is a book worth reading: light, entertaining and extremely polished.