The Window by Glenn Rolfe
After the divorce of his parents, James Curry finds himself torn away from the town he grew up in, his friends, the girl he loves, saddled with a stepfather he can't stand....and worst of all, missing his dad.
When his mom finally caves in, and allows him to spend the last month of summer vacation back home with his father, James is thrilled to see his old friends, his girl, and his dad.
But things haven't been going well for his father. Not for a while. In ways that James finds increasingly disturbing. Turning to his friends for help, he unwittingly puts them all in mortal danger from forces none of them can accept or believe.
Rolfe has written an engaging coming of age horror novel, peopled with believable, fully fleshed characters, a wonderful retro 80's feel, and unrelenting terror that he unleashes on the reader with both barrels.
Highly recommended.