Last year, with Sister Orchid, Nellie McKay delved into the standard songbook as if rummaging through a costume trunk - but she wasn't fooling, not really. She offers another master class in wry sincerity with Bagatelles, accompanying herself on ukulele, piano, Fender Rhodes and Theremin, on such indelible American anthems as "Zip-a-Dee-Doo-Dah" and "The Best Things in Life Are Free."