The Hungry Moon, by Ramsey Campbell, 1987
This book has been languishing on my shelves for some time and I finally got around to reading it recently on holiday.
Plot Summary: The small, isolated (and, needless to say, fictitious) town of Moonwell lies in England's Peak District, where it receives less sunlight than any other place in England. Like many other towns in the area, Moonwell practises the ancient custom of well dressing, when a local cave is annually decorated with Christian images. Like all of these practices, this is rooted in pagan religion, but the origins of Moonwell's custom are far more sinister than most. Just how sinister starts to become apparent when an American evangelist comes to town to bring his version of Christianity to its people and end the "evil" custom. As almost all the townspeople are converted and the evangelist prepares to descend into the cave, the darkness increases, the moon looks hungrier, and the town begins to disappear from the world. Meanwhile a handful of people who refuse to follow the evangelist fight to prevent the impending disaster in the face of bigotry and hatred from the newly-minted "Christians".
Opinion: I'll be completely honest. This wasn't the best novel I've ever read. Having said that, it was far from being the worst. It's a competent work of imagination and, although Campbell isn't the best writer technically, it's also reasonably well-written. It held my interest throughout and it made me want to know what happened next, which is always a good recommendation. The characterisation is reasonable, if a little stereotyped (e.g. the sceptical journalist, the strange old local who knows all the secrets of the town, the ageing hippies), and I generally found myself caring about the characters I was supposed to care about. Campbell narrates his story from the point of view of a number of characters, and this does by and large work. The ending, although effective, is a bit abrupt. There are a few unlikely moments. The speed with which almost the entire town converts to the evangelist's version of Christianity (which is terrifyingly portrayed in all its bigoted glory) and turns on the "unbelievers" seems exceptionally unlikely. Headmasters can't just sack teachers in Britain - they're employed by the local education authority (in this case, the county council), not the town. And why on earth do British people in the novel continually refer to torches as flashlights? Overall, however, I can recommend The Hungry Moon as an entertaining and creepy read.
Use in Call of Cthulhu: The Hungry Moon is not strictly a Mythos novel, but it does have a very Mythos feel and with a bit of adaptation could be run as an effective CoC scenario. An avatar or servant of Shub-Niggurath is an obvious choice for the protagonist.