The Farseer Trilogy #1: Assassin's Apprentice book by Robin Hobb

In a faraway land where members of the royal family are named for the virtues they embody, one young boy will become a walking enigma.

Born on the wrong side of the sheets, Fitz, son of Chivalry Farseer, is a royal bastard, cast out into the world, friendless and lonely. Only his magical link with animals - the old art known as the Wit - gives him solace and companionship. But the Wit, if used too often, is a perilous magic, and one abhorred by the nobility.

So when Fitz is finally adopted into the royal household, he must give up his old ways and embrace a new life of weaponry, scribing, courtly manners; and how to kill a man secretly, as he trains to become a royal assassin.