The Distinctive Baritone said: "I'm afraid I must slowly and awkwardly raise my hand here and also say that I had always thought that although they work closely with the composer/lyricist on the show's structure, technically the book writer's main duty is writing the dialogue between the songs. And let's be honest, until sung-through musicals became more common, that was essentially their role.
Can anyone name a sung-through musical in which the credited book writer was neither composer nor lyricist? There are I imagine few if any, because in sung-through musicals, the composer and lyricist take on the structural/storytelling duties a book writer would normally have.
So frankly, I'm with Isherwood in this one. If the Dramatists Guild would like to "update" the definition of a book writer due to the popularity of sung-through musicals, that's fine. But by the traditional definition, Hamilton has no book, because it's sung-through."
I'm very confused by your logic. I don't see the rise of sung-through musicals as relevant here, except in the sense that Hamilton has been the catalyst for this discussion. Yes, the lyricist and book writer of sung-through musicals are often the same person, but that does not mean they aren't doing both jobs. The same could be said for the music and lyrics of any musical. And yes, the book writer works closely with the composer and lyricist to structure the show, but how does that mean that it's suddenly not their job to structure the show? It's a collaboration. It goes the other way too.
What exactly do you mean when you say "traditional definition"? How can you claim that the TRADITIONAL definition of a book is only the spoken words, when any IBDB page of a sung-through musical, any member of the Tony committee, and any person who has written a musical will ALL tell you the exact opposite?