Incredibly well-researched, The Agony and the Ecstasy was clearly a labor of love for Irving Stone. Following Michelangelo's life from his first truncated apprenticeship with Domenico Ghirlandaio (a workshop that produced frescoes, not sculpture) to his acceptance into the house and sculpture garden of the Medici, to his work in Rome for cardinals and popes, to his death, the book is an almost exhausting chronicle that occasionally required patience for me to finish. In the end, I think it was worth it.
Stone lived in Italy to research the book, worked in a quarry for a time, and studied under a marble sculptor, as well as collaborated with a Canadian who specialized in researching period stone-carving tools. He also translated nearly 500 of Michelangelo's letters, which I assume gave him an "ear" for the artist's written voice, and brought to life some of Michelangelo's inner workings (Michelangelo's sonnets, while published and celebrated, are kind of hollow and sentimental, I think).
This is historical fiction that tends toward dramatized non-fiction in some ways--that is, a precursor to the style of Hilary Mantel's Wolf Hall and Bring Up the Bodies. There is no satisfying story arc here the way there is in fiction, and no interesting character change beyond the normal growth and learning that happens to a human being who lives to be eighty-eight years old. It's more like you're doggedly following alongside Michelangelo, immersed in a detailed imagining of his life, than reading a novel. The fictional, invented (or maybe exaggerated in the last case) parts are three heterosexual romances: Contessina de' Medici (a love that they never act on, or speak of until her deathbed), who is the daughter of Lorenzo the Magnificent; Clarissa (the mistress of Marco Aldovrandi, with whom he has a sexual relationship in the book); and Vittoria Colonna (who has a strong platonic bond with him but chastely rejects his advances). Stone avoids stating outright whether Michelangelo had a sexual relationship with Tommaso dei Cavalieri, the man he wrote dozens of passionate sonnets to, but I think makes it clear that the love is a powerful and charged one. Maybe because the book was published in 1961, Stone thought readers weren't ready for a homosexual or bi-sexual Michelangelo.
Stone excels at imagining Michelangelo's working process and his relationship with his family and surrogate Settignano family, and creates for us highly realistic period settings in Rome and Florence. The title of the book is a giveaway of what much of the "story" is: a creative person nearly possessed by the passion he feels for his art and the visceral connection he feels to his marble (which is almost a substitute for a romantic and sexual passion), and how a solitary, brusque, and sometimes melancholy artist might have interacted with family, friends, and patrons. I love the way Stone "gets" both the single-minded passion of an artist, and the way ordinary people are sometimes cowed by it. Michelangelo has what looks to us to be a simple, pure devotion to, and pride in, his work--more important to him than people and sometimes politics--but it gives him the reputation of having a terribilità (powerful will, angry force) that carries into the perceived aesthetic of his art.
The pacing falters a little toward the end of Michelangelo's life: it speeds up beginning when Cosimo I de' Medici comes to power (1537) and kind of hop-skips to the end of Michelangelo's life (1564). Perhaps there was a paucity of primary source material for those later years, or maybe Stone just collapsed under the weight of his own manuscript at that point.