Covenant — Jacqueline Farrell

Fifty, English, and a professional crone, Sophronia Sheridan is on a tour of the USA when she comes across a juvenile witch caught up in a dangerous relationship with the youngest member of the local undead community. Driven to help, against her better judgment, she draws the unwelcome attention of the head vampire, who has plans to use her skills for his own ends. But Sophronia is more than capable of looking after herself...
This novel was first published as SOPHRONIA AND THE VAMPIRE.