Seventeen years ago, I read Colm Toibin’s fictional biography of Henry James called The Master. It was the best book I’d read in a long time, the marvelously imagined life of a great writer. It got rave reviews. Now, with The Magician, Toibin has done it again, this time with life of Thomas Mann, a work equally persuasive, intimate, and marvelous in its power to compel belief.