From Amazon.co.uk:
The setting is quiet Newton, Massachussetts, where nothing ever happens. Nothing, that is, until two months after Henry Dudlow's 13th birthday, when his neighbor, Mr. Hanley, suddenly starts to look... different. While everyone else sees a balding man with a beer belly, Henry suddenly sees a nasty, bilious, rage-filled demon. Once Henry catches onto the real Mr. Hanley, he starts to see demons all around him, and his boring, adolescent life is transformed. There's no more time for friends or sports or the lovely Sally Freeman - instead Henry must work his way through ancient texts and hunt down the demons before they steal any more innocent children. And if hunting demons is hard at any age, it's borderline impossible when your parents are on your case, and your grades are getting worse, and you can't tell anyone about your chosen mission.

Ex-Liverpool City Library book.

Left at the book exchange.

Release #10 for the 2020 Spook-Tacular Halloween Challenge.