Original: Thus, a musical such as Fiddler on the Roof has a composer (Jerry Bock), a lyricist (Sheldon Harnick), and the writer of the "book" (Joseph Stein).
Simplified: Sometimes these two are by different authors, e.g. the musical Fiddler on the Roof has a composer (Jerry Bock), a lyricist (Sheldon Harnick), and the writer of the "book" (Joseph Stein).