How did a dandified dilettante, after two decades of living (in good part) off his father-in-law’s money and his own personal charm, turn himself, at the age of fifty, into one of the formative influences on the art of the second half of the twentieth century?

Leo & His Circle: The Life of Leo Castelli by Annie Cohen-Solal (Alfred A. Knopf, 525 pages) taps into the life of the legendary art dealer who can be credited (or blamed) for having single-handedly created the art-as-commodity phenomenon that bolstered the careers of many of the 20th century’s brand name artists...