Books like Assassin's Apprentice / Royal Assassin

Young Fitz is a bastard son of the noble Prince Chivalry, raised in the court of the Six Duchies by his father's gruff stableman. He is ignored by all the royalty except the devious King Shrewd, who has him secretly tutored in the arts of the assassin. For in Fitz's blood runs the magic Skill--and the darker knowledge of a child raised with the stable hounds and rejected by his family. For longer than anyone remembers, the Farseers have ruled the storm swept kingdoms of the Six Duchies--using the wild and perilous magic of the Skill. But now, as the barbarous Red Ship Raiders ravage the coasts and sleepwalkers prowl the heartland, Fitz is growing to manhood. Soon he will face his first dangerous, soul-shattering mission. For Fitz is a threat to the throne ... but he may also be the key to the survival of the kingdom.