Primal Instincts by Melissa Schroeder

As Alpha of the Lonestar Wolf Pack and Mayor of their small Texas town, Jacob Sanderson has more than enough on his plate to handle. When women from his pack start getting killed, he knows his best resource for solving the murders is Sheriff Alexandra Littlefoot. Now, if he can keep his hands off her, everything will be fine.
A bad shooting and the loss of a partner caused Alex to leave San Antonio. She sees being sheriff of the small town as penance for her sins. Working with Jake isn’t exactly what she would call ideal, but he insists on helping with the investigation. Close working conditions make it impossible to ignore their attraction, and one kiss leads to more than either of them expected.
After one night together, he knows this is no casual encounter, and walking away is impossible. He knows she’s his mate, and while she isn’t a wolf, he knows she has some connection to the pack.
As secrets rise to the surface, the killer’s violence escalates and threatens to destroy not only their fragile new love, but the entire Lonestar Pack.