Age of Assassins (The Wounded Kingdom, #1) by R.J. Barker

To catch an assassin, use an assassin...

Girton Club-foot, apprentice to the land's best assassin, still has much to learn about the art of taking lives. But his latest mission tasks him and his master with a far more difficult challenge: to save a life. Someone, or many someone's, is trying to kill the heir to the throne, and it is up to Girton and his master to uncover the traitor and prevent the prince's murder.

In a kingdom on the brink of civil war and a castle thick with lies Girton finds friends he never expected, responsibilities he never wanted, and a conspiracy that could destroy an entire kingdom.