Demon by Rinda Elliott

Demon Callan Ambrose wants a soulmate more than anything but is sure he’ll never find his. One of his brothers is mated and soulmates are rare, so the odds are against him. Then he takes a job protecting Lonnie Russell, a creative writing professor who is quirky, and sweet, and just…perfect. But someone is terrorizing Lonnie with threatening notes.
Lonnie has always been able to see the preternaturals in the world and he’s always been fascinated with them. Especially demons. He even has an entire room dedicated to them. So he’s thrilled when his new bodyguard turns out to be one. Callan is big and gruff, his grumpy exterior a mask for someone who’s really a cuddle monster. And the gifts? He sure would like to know what that’s all about.
What started out as vague threats quickly escalates and when they realize what they’re up against, Callan is determined to keep his newfound treasure safe.

Demon is a long MM paranormal romance novella with a grumpy/sunshine pairing, traditional demon courting, and plenty of steam. It’s the third in the Mystic Guardians series, but can be read as a standalone.