Village Library Demon Hunting Society by C.M. Waggoner

Librarian Sherry Pinkwhistle has a knack for solving murders – lots of them. Sometimes she’s concerned by just how many killers she’s had to track down in her quiet village, though none of her neighbours seem surprised by the rising body count…

But when someone close to Sherry ends up dead, and her cat becomes unexpectedly possessed by an ancient demon as irritating as it is infernal, Sherry decides that it’s time for action.

It will be a lesson for murderers and demons alike:

Never mess with a librarian.