Dark Moon Defender by Sharon Shinn

Dark Moon Defender is the third in the Twelve Houses books by Shinn. As I read this series, it seems to me that they fall somewhere between the "buddy movie" and the "questing fellowship". The band of characters established in the first couple of books just float in and out of this one, as needed. The main focus is on Justin, a King's Rider with a rough past. He was the illegitimate son of a prostitute, lived as a thief on the streets, but met his match when he tried to rob Tayse, a King's Rider. Tayse saw something in young Justin and recruited him to the Riders, and now Justin is fiercely loyal to the Riders, King Baryn, but mostly Tayse. "If he ever had to chose between saving the king's life or saving Tayse's, he would save the king's, but only because it was what Tayse would expect of him."

Justin is sent on a clandestine mission to the town of Neft, which is close to the Shrine of the Pale Mother, where Coralinda Gisseltesse runs a cult which persecutes and kills any mystics it finds. Coralinda is also suspected of plotting rebellion against King Baryn with her brother, Halchon, so Justin's instructions are to keep an eye on things and report back any suspicious behavior.

One night, Justin saves a young initiate of the cult from being assaulted by a drunken thug, and finds himself inexplicably drawn to her. He begins to meet with her, telling himself that it's another way to gather information about Coralinda's plots, but it's obvious there's a bit of romance in the air.

The girl, Ellynor, is a member of one of the Lirren clans, who was sent to accompany her cousin, Rosurie, to the shrine, where her family hopes to keep Rosurie out of trouble, sending her away from a boy from a clan unallied with her own (shades of Hatfield and McCoy here). It's become apparent to Ellynor, though, that girls who come to the shrine are not allowed to leave later. What's not so apparent to her, but is apparent to anyone who's been following the series, is that the gift of healing that she has naturally, and some of her other abilities, mark her as a mystic. It's only a matter of time before someone in the cult figures it out and she is exposed, probably to be burned at the stake for her "crimes."

So, we have an undercover King's Rider who falls in love with a feisty, naive mystic. What could possibly go wrong? Shinn tells a great tale, as always, and I'm sure you'll enjoy it.