The Crystal Cave, published in 1970, is Mary Stweart's first book in the Merlin Trilogy. It is an incredibly well written and emotional book, very factual and grounded in historical detail and fact and also incredibly imaginative. I read it when I was in junior highschool, and was swept for the first time into the mistly lands of Arthurian Legend.
Merlin is a small, dark boy. He is a bastard, the son of a princess. He does not know who his father is, and his mother will not say. Legend soon spreads that his father is Lucifer himself, some dark demon, or fey visitor who seduced his virgin mother. The bastard of a princess, small for his age, quiet and overly bookish besides, Merlin is not treated well. He is, for the most part, forgotten. When he is remembered, he usually fares badly. He does manage to recieve and education, and enough freedom to find his own tutor... the hermit, Galapas.
Galapas instructs young Merlin in Medicine, in Geography, in Languages, and in Magic. He gives the boy the tools he will need to someday advise the High King of Britain.
When Merlin's Grandfather dies and Merlin's servent and friend is killed in punishment, Merlin decides to leave. He is picked up by Ambrosius' Men, and taken across the sea to meet the great man. The encounter changes Merlin's life, and history, forever.