The Book of Atrix Wolfe by Patricia A. McKillip.
First sentence: “‘The great mage moves,’ the mage Danicet said twenty years later at the mages’ school in Chaumenard, ‘from moment to moment, from shape to shape, to meet the constant, ever-changing needs of life.'”
It’s a rather misleading first sentence, since neither Danicet nor the mages’ school is of very great prominence in the story; on the other hand, the novel does open with a rather gripping prologue, so by the time you read this sentence, you’re already hooked.
As I am, on Patricia A. McKillip novels. I think my favorite so far is The Bell at Sealey Head, but I’ll clearly have to read lots more to be certain.