Hollow by Karina Halle

Ever since she was a child, Kat Van Tassel knew that her parents had arranged for her to marry her best friend, Brom Bones, when she became of age. That was until Brom mysteriously vanished from the secretive village of Sleepy Hollow when he was just a teenager.

Now, at nineteen, Kat is fulfilling the tradition of attending Sleepy Hollow Institute, a shadowy and prestigious school that is run by her powerful family. There she starts to fall for her new teacher, the enigmatic, smart, and much older Ichabod Crane, whose fascination with dark magic and the occult brings out a new side of Kat—in more ways than one.
Until Brom Bones suddenly appears in her class. Her childhood friend doesn’t remember what happened to him in the four years that he’s been gone, all he knows is that he’s back in Sleepy Hollow for good. The only problem is, he’s not the same man that Kat knew. Cocky and brash as a youth, this Brom is violent and prone to bouts of madness. It doesn’t help that his reappearance coincides with gruesome decapitations that are plaguing the town, with rumors swirling about a vengeful ghost called The Headless Horseman.
Determined to get to the bottom of the murders and Brom’s volatility, Crane takes Brom under his wing to study him, and they begin to grow close, just as they do with Kat. But the more that the threesome submits to their sexual desires on the path to the truth, the more dangerous Sleepy Hollow becomes. The Headless Horseman might claim them as his next victims…if something even more terrifying doesn’t get to them first.