Dragon Slayer (The Empty Crown, #1)

Ingram is a coward and weakling—at least according to his father, the king, and the royal court. He cannot use a sword, he faints at the sight of blood, and even his brilliant abilities as a strategist are not enough to overcome his failings. When his father loses a bet to the notorious Lord Mallory over the matter of a dragon slaying, he pays his debt by ordering Ingram to marry him.

Then his father reveals that he is putting Ingram to a greater purpose, giving Ingram one last chance to prove he is not worthless. All it requires is betraying his new husband.