“Moon River,” “Laura,” “Skylark,” “That Old Black Magic,” “One For My Baby,” “Accentuate the Positive,” “Satin Doll,” “Days of Wine and Roses,” “Something’s Gotta Give,” — the honor roll of Johnny Mercer’s songs is endless. Both Oscar Hammerstein II and Alan Jay Lerner called him the greatest lyricist in the English language, and he was perhaps the best-loved and certainly the best-known songwriter of his generation. But Mercer was also a complicated and private man.