IceCreamBenjamin mentions "Musicals where the majority of the book is underscored isn't sung-through, but is rather more in the realm of an operetta."
ICB, I'm not familiar with the apparently technical term "underscored." Could you explain?
Also, how are you defining an operetta? To me, an operetta is mostly, if not all singing, but the music isn't as "heavy" as an opera score. Like the question of whether a musical is actually an opera, the difference between opera and operetta is, to me, in the eye of the beholder.
Audrey, the Phantom Phanatic, who nonetheless would rather be Jean Valjean, who knew how to make lemonade out of lemons.