Saved By the Rat by Kaje Harper

Robin
I’ve been managing my uncle’s secondhand building-supply center long enough to know that I need to make an impression on new employees first. I’m five-foot-two and skinny and look younger than twenty-five. I start with the power suit and the power attitude, until I get the upper hand. Problem is, with Alaric, I’ve been bossing him around wearing the stupid tie and shiny shoes for a week, and I don’t think he’s impressed yet.

Alaric
The kid running this store doesn’t know that dangerous artifacts were dumped here in the clearance of a local sorcerer’s house. Robin’s human. To him, magic’s just a trick and a joke. My familiar Harry and I know better. If I don’t find the missing grimoire bound in human skin before someone less scrupulous does, we’re all in trouble. I need to ignore Robin’s cute ass and keep searching, although that ass gives me an idea for a distraction if I need one.

Saved by the Rat is a standalone 17,000-word story set in the Necromancer universe in modern times. (content warning for violence)