The Crystal Cave: Mary Stewart

This story is about the childhood of Merlin. His mother, a princess falls in love with a prince from an enemy territory. She refuses to tell anyone who his father is and refuses to marry. Eventually she enters the convent and Merlin tries to run away, but is kidnapped by his father, which seems like fate. Once his maternal grandfather's kingdom is captured he goes to see his mother who gets sick and dies before she is reunited with Merlin's father. Merlin's father also dies and his uncle uses him to impregnate the queen before the book ends. A lot more happens obviously, but it is a long book.