In Lyrics on Several Occasions (1959) Ira Gershwin described the art of the lyricist as that of fitting words "mosaically" to music, an art that required "the infinite patience of a gemsetter." To answer that perennial question, whether the words or music came first, it was usually the music, or at least the musical germ of a song. "I hit on a new tune," George Gershwin once explained, "and play it for Ira and he hums it all over the place until he gets an idea for a lyric. Then we work the thing out together." Working it out meant finding the precise syllables, words, and phrases that fit the notes of a melody. Sometimes it took Ira all night to find a single word. Once he had to check into a hotel room for three days to find a setting for a tricky string of staccato notes in...