Just edited the question to include a detail that I might have missed. I'm fairly sure the moon had something to do with the creature coming out of the well, and may have even featured on the cover of the book.

@Emsley Night world seemed promising, but after reading the synopsis on Wikipedia I'm fairly sure that's not it. In the story I read there were fewer traditional fantasy elements - no magic swords or demon big bads as far as I can remember.

Just had a quick look through the synopses on the Wikipedia page and at the titles of the stories, but none of them sound similar to what I remember. The character of Repairman Jack certainly doesn't ring any bells.