Le Morte D'Arthur
For the first part of Arthur's reign, Merlin's the go-to guy for advice and information. After all, it's he who engineers the whole sword-in-the-stone thing to get England the king it needs, and it's he who confirms Arthur's identity. He's also the one who attempts to rescue the kingdom from the threat of Mordred by advising Arthur to kill all the boys who are born on his birthday.
If you're a little uncomfortable with that last one, you're not alone: Merlin is, in many ways, a deeply flawed mentor. He can see into the future, but he's usually powerless to change it or to help characters deal with it, which means that the information he provides, like the whole Mordred's-going-to-kill-you thing, sometimes does more harm than good. On the other hand, though, Merlin does help Arthur to get a magical sword that makes him invincible in battle. Now if only he could prevent Morgan le Fay from stealing it…