This, as its title implies, is a sourcebook focusing the Cthuloid aspects of the Freeport setting, which have, of course, been there ever since the original adventure trilogy. We get eight evil cults here, any of which could be easily transported to any other fantasy RPG setting - since there are no rules here, the RPG in question doesn't even have to be a variant of d20.
The cults themselves are mostly chaotic evil, with two or three exceptions. Four have more or less directly Mythos inspired origins - Hastur and Yig are, of course, included, and the other two (while never named as such) are Nyarlathotep and Morrigan. A fifth is based on the Forgotten Realm's version of Loviatar, here envisaged as a goddess of BDSM. For the remaining three cults, we have vicious pirates, seekers after forbidden knowledge, and demon-worshippers. Each cult has its own distinct personality and theme, giving a variety of different forms of evil and avoiding a feeling of "sameness".
At 145 pages, this is a large book, and with none of that taken up by stat blocks, that means that each cult can be fully fleshed out. We have their history, mythology, their aims and objectives, rites and rituals, and so on. Furthermore, the actual cult as it exists in, or near, Freeport is described with detailed write-ups of its NPCs, and room-by-room descriptions of whatever passes for its temple - each with a detailed floorplan.
The book is black and white, with high quality artwork. Much of the material in it is pretty gory, although it falls short of explicit detail. Most of the cults, in short, are truly loathsome, and we are left in doubt as to why. Each is also accompanied by a resume of a four-part series of scenarios (you'll have to provide the details yourself) showing how PCs can encounter, fight, and hopefully destroy each cult as they rise in level.
In short, if you're looking for some worshippers of evil and insane gods to include in your game, this is a useful resource, with plenty of original ideas. Although written for Freeport, it could easily be transplanted into any setting where the cosmology fits. The statless nature may be a problem for some, though. You'll have to figure out stats for NPCs, and also work out what magical powers (if any) the evil gods give their followers, and I could see that for some GMs, especially of rules-heavy systems, that could be quite a pain.
But if all you want is ideas, here are some truly memorable and ghastly villains to bring a taste of horror to your fantasy game.