After seeing the film NIGHTBREED, I promised to read the novel upon which it is based, titled 'Cabal'. I would say the novel is a better novel than the film is a film.
Barker is pretty good at describing people's feelings in writing, but not good at getting actors to express those emotions through their performances. The characters also have a lot of internal struggles that are presented as thoughts in the novel. A film has to either find a way to express those thoughts externally, which is trickier, or resort to a narration track, which usually feels lazy in movies.
The film follows the novel fairly faithfully, although Barker does get that some things need to be changed for a film adaptation and he changes those things. The main one being that in the novel he leaves it ambiguous as to whether Boone is a serial killer for quite some time. In the film they reveal it right away that his psychiatrist, Decker, is really the killer and framing him. When you see actors, less is up to your imagination, so the audience will clue in quicker that a character is untrustworthy. So it makes sense to not bother trying to make that a secret since it's obvious from Cronenberg's performance that he is sinister.
Also, the part where God tells Boone his destiny and Boone responds by jizzing on God's face was probably too difficult to film before CGI, so they left that out. Maybe Barker will pull a George Lucas and get that added back in later.
I'd say the novel is 80% from the perspective of Boone's girlfriend, Lori figuring things out. The film puts a bit more of the story back on Boone, which is more cinematic and the right choice.
All and all, it's a good novel. It has some of the same weaknesses as the film. I wish we got a bit more sense of community and belonging from the Nightbreed. It's like Boone's barely accepted into their society before it gets destroyed. But I like this story. Good read.