Bob, your idea makes sense to me. I have no doubt that "book" is proper, and would have thought that "libretto" is not -- but on googling, I do find "Broadway" and "libretto" used together quite often.

Scanning through the sites, my sense is that "libretto" is used to mean the detailed story-line of a musical, and "lyrics" to mean the specific words sung when the characters break into song. For example, "In 1948 Loesser was approached by fledgling Broadway producers Cy Feuer and Ernest Martin to write music and lyrics to George Abbott's libretto for an adaptation of the classic Brandon Thomas play Charley's Aunt." But I am very tentative on this. Comments?