Liza's Headband and neolights -
Let me try yet again to be more clear. Because sung-through musicals are still a relatively new thing (new as in having been mostly a rarity until the British poperetta explosion in the 80's), many people still think of the "book" for a musical is merely the dialogue, because that's pretty much what it used to be. The story/structure was usually a collaboration between the composer, lyricist, book writer, and even the director or producer (Hal Prince himself came up with the idea of the Emcee in Cabaret). However, to view the book writer as merely responsible mostly for the dialogue is an archaic way of defining the book of a musical in light of the increasing popularity of sung-through musicals. I agree with Doug Wright and the Dramatists Guild that the definition of "book writer" needs to be updated.
Some classic "traditional book musicals" (a term that has been created by theatre scholars for good reason): Fiddler on the Roof, My Fair Lady, Carousel, 1776, She Loves Me...okay, really, anything written prior to the 1980's except for Jesus Christ Superstar and a few others. The structure is song/dialogue scene/song/dialogue scene pretty much the whole show.
Some modern "traditional book musicals): Urinetown, The Producers, Wicked, etc.
Do I have a source for this "old" definition? Yes - over twenty years of theatrical study, practice, and experience. I make my career in the theatre as an actor, director, and teacher so I do know what I'm talking about.
So for the last time, yes, the traditional definition of a book writer for a musical needs to be updated. And thanks to Doug Wright and the Dramatists Guild, it has been. Yay. The end.