You can’t be too careful with Walt Whitman, or maybe you can. In reading him aloud, much less setting him to music, he can come off sounding like Kahlil Gibran or a self-infatuated guy with a ponytail unless you convey the hurry and delirium of his words—his long meter and barbaric yawp. Oh, wait a minute, the guy with a ponytail is Kurt Elling, a singer likely to transform even a ballad from the 1940s into a song of himself, if left to his own devices. Surprise—blessed with an actor’s presence and lovely enunciation that lets him segue from recitation to song without a hitch, Elling is the best thing about Fred Hersch’s Leaves of Grass. (Kate McGarry, used coloristically for the most part, is in over her head.) No composer has ever quite gotten Whitman right: not Hindemith, not George Crumb, not John Adams, not Roger Sessions. And not Hersch, whose art-song settings could stand more “reckless heaven-ambitious peaks” and “gossip of flames”—I quote Whitman, but maybe I just mean more Coltrane.. Still, Leaves of Grass is exquisite for the orchestral munificence Hersch draws from a seven-piece ensemble, and even though exquisite might not be right for Whitman, “The Sleepers,” where Hersch effects urgency by having the horns vamp a pedal point and forcing Elling into a strained but pleasing near-falsetto he almost never uses, is close to ideal. Mike Westbrook and Phil Minton’s “The Fields” and “I See Thy Form,” from Westbrook’s William Blake (1981), remain the hybrid genre’s gold standard, but “The Sleepers” comes close.