15-10 years ago I read a horror/fantastic-thriller novel. It took place in a smallish American town (don't remember which part of the US). The protagonist could be maybe between late twenties up to forty-ish. He had a dog (I think) and a couple of friends in town but seemed kind of a loner.
The town itself was strange, with an interdimensional portal to another world that seems to influence most of the people in town. Some characters (I think bad-guys, but maybe greyish) had an encounter with something bordering to cosmic horror. I remember reading that one of the characters in the encounter didn't want the creatures to slip back to Earth. I remember that they found the creature's eggs and that the mature creatures could fly.
The police in town is corrupt, connected to the interdimensional portals that exists. But at least one senior officer is at least coldly friendly with the protagonist. In the way of "I like you, but if you go down this road I can't protect you"-kind-a-way. So not 100% a dark character but not a good guy.
I first suspected the Odd Thomas books by Dean Koontz but it doesn't ring a bell when I read the plot synopsis. The style of the book might be a little Koontz-ish though.