The subtitle of Rachel Cohen’s luminous biography of legendary art critic and historian Bernard Berenson is “A Life in the Picture Trade.” This is an apt characterization given Berenson’s role in building some of the greatest private Renaissance art collections of America’s Gilded Age (most notably Isabel Stewart Gardner’s). But it’s also one that Berenson would have looked upon with more than a little horror, since admitting he engaged in any kind of “trade” would conflict with his carefully constructed self-image, worked out over a lifetime of secrecy and reinvention.