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A Fistful Of Charms (The Hollows Book 4) by Kim Harrison

Rachel gets a frantic call for help from Jenks the pixie:  his son Jax has gotten into trouble with Rachel’s ex-lover Nick, a demon-summoner and thief of magical artifacts. Apparently Jax is lost somewhere in Michigan after Nick was taken prisoner by a group of werewolves who have a compound on a private island.  Jax and Nick stole an artifact that the werewolves want. 

So Rachel casts a spell that makes Jenks a full-sized man and the two of them go after Jax and – with some reluctance on Rachel’s part and great resentment on Jenks’s part – Nick. This involves getting scuba gear, swimming to the island, evading werewolves on the island, and other adventures. 

Once they return to mainland Michigan, the artifact that Nick has stolen is revealed to be such a big deal that Ivy has to come up to help them. Nick is revealed more and more to be a dirtbag con artist.  Rachel is tempted more and more to evolve her relationship with Ivy towards love and sex. Jenks steals every scene.  Great fun.