Dire Bound (The Wolves of Ruin Book 1) by Sable Sorensen

Only the worthy survive the Bonding Trials. She'll risk her life - and her heart - to be one of them.

Meryn Cooper has always hated the Bonded, elite warriors who form mental links with the massive, vicious direwolves they ride. While they live in luxury, Meryn struggles to keep her family out of poverty. When her little sister, Saela, is kidnapped - stolen across the border by the immortal monsters her country has spent centuries fighting - Meryn's world falls apart.

Desperate to cross the front and save her sister, Meryn enlists in the army and is thrown into the deadly Bonding Trials, where any mistake will cost her life.

Now Meryn must survive four months of training at the castle. She is bound to a feral direwolf who refuses to communicate. The other trainees would love to spill her common blood. And her cold and beautiful instructor, Stark Therion, is eager to punish any weakness.

Everything is a competition, and everyone is out to get her - everyone except the dangerously handsome crown prince, whose attention adds another target to her back. In the castle, every smile hides a knife... and the halls hide dark secrets.

It's bond or bleed. Duel or die. Failure is ruin.