Unraveller, by Frances Hardinge

In the land of Raddith, some people grow "curse eggs" within them.  Given a strong enough grudge, the egg can hatch and its bearer will lash out and lay a curse upon the target of their anger.  Society has learned to deal with the cursers by locking them away, but the cursed are never freed.  Until now.

Kellen has found himself in possession of the ability to unravel curses.  But the power comes with caveats:  he must know who made the curse and why the curse was made.  For now, he uses the power as a means to make a modest living, but as word of his potential gets around, the power makes Kellen a target.

Someone is rescuing accused cursers and abducting people with unhatched curse eggs.  And that same someone has now cursed Kellen.  All he knows is that the person is somehow connected with a shadow organization called Salvation that lurks in the dangerous and untamed wilds outside of human civilization. So, he joins up with his friend Nettle, her cursed brother Yannick (who lives life as a sea gull), and the help of a warrior and his bonded horse monster, and they head into the wilds to find out who is doing all of this.  Along the way, they uncover the mystery of where curse eggs come from and why Kellen is able to unravel curses.

Its a long and very complicated fantasy adventure with a delightfully original internal logic and lots of twists and turns.  I particularly enjoyed the logic of curses, an idea that combines magic with some behavioral observations about the way anger and grudges can consume a person and about how unsatisfying revenge truly is.