Prompted, perhaps, by the publication, in the early seventies, of Alec Wilder’s groundbreaking study, “ American Popular Song: The Great Innovators 1900-1950,” the old songwriters have come to have a new presence, and their songs even a collective brand name: the American Songbook. Now those recordings, and the songs they illuminate, are everywhere. In the mid-nineteen-seventies, you had to haunt London record shops to find Ella Fitzgerald’s Gershwin or Cole Porter albums. Back in 1976, the incomparable drama critic Kenneth Tynan wondered in his diary when someone was “going to take a deep breath and declare that, at some time in the thirties, the serious music tradition finally withered, curled up and died of sterility and malnutrition and that the greatest composers of the twentieth century are Berlin, Rodgers, Porter, Kern, Gershwin, et al.” This view, bold enough at the time to be fit only for a diary, has by now become commonplace.