The Leper Window — Frazer Lee

Do not gaze into its secrets.

Daniel Gates's last assignment involving a rare demonic book left him plagued by hideous nightmares, which are about to get even worse. When a page is stolen from Choronzon's Grimoire, Daniel is sent to the wilds of North Wales to retrieve it for his mysterious client. But the stolen page contains dark, occult secrets, and he finds to his peril that he is not the only one desperate to find it. Daniel's journey takes him to the very brink of madness and to the demons that await -- beyond the Leper Window.