I’m getting different definitions from different sources, so I hoped someone could clarify. In a musical, you have the spoken words, the sung words, and the actual music. Some combination of these is the* book* and the* libretto * … or they might be the same thing. And the music alone might be the score, or the music and the sung words are the score.
What’s what?
As far as the “score” is concerned: For the singers, the score might be just the singers’ part, possible with a piano reduction. For the pit orchestra, the score would be the individual instrument’s part. The conductor would have the complete score, including all the parts. But there’s no hard-and-fast rule for this, so it varies a lot.
The ‘book’ is the non-sung words, the dialogue.
In my experience, which is not necessarily definitive, the word ‘libretto’ is used more often (though not exclusively) in the opera community, whereas ‘book’ & ‘lyrics’ are used almost exclusively in the musical theater (broadway) community.
“Book” denotes the story & dialogue; “lyrics” are just that - song lyrics. A “book” can sometimes be assumed to include “lyrics,” especially when they’re written by the same person, but not always.
The “score” consists of everything created by the musical composer, and while he/she often does not write the lyrics to the songs (although they sometimes do), the lyrics are an inextricable part of the finished product, so they can also be considered part of the score for practical purposes - just not for “credits” purposes.
Essentially, the different terms exist so that people can be easily and accurately credited with their part of the creative process, as in:
Book by Jim
Lyrics by Sue
Score by Pat
That way everyone knows who did what.
On review…I feel I have only stated the obvious… :o
Often, the writer of the book for a Broadway show is not the lyricist (though it does happen, and seems to be more common nowadays than in the past).