Hell Hound (Jane Doe Chronicles #1) by M.J. Sylvester

SPFBO6

A foul-mouthed bounty hunter and assassin, Jane Doe is not your average witch. Working for the ruling magical class in Britain - the Merlins - she takes on the jobs and creatures that other members of the magical community wouldn't touch with a six-foot spell staff. As rumours spread that a powerful being wants to free a horrific hound from its prison on Dartmoor, Jane and her assistant, Dawn, are tasked by their Handler to find the perpetrator and stop them using any means possible.