What artist was the subject of book The Agony and Ectasy?

The Agony and the Ecstasy is a biographical novel written by Irving Stone and first published in 1961. The novel tells the story of the Italian Renaissance painter and sculptor Michelangelo Buonarroti. The novel was adapted into a 1965 epic historical drama film of the same name, starring Charlton Heston as Michelangelo.