Which “book” do you mean?
In musical theatre the book is the dramatic text supporting the musical numbers. Arthur Laurents wrote the stage book. Irving Shulman wrote a prose novelisation adapted from the Broadway musical. Shakespeare's Romeo and Juliet is the earlier dramatic source. If a teacher or director says to read the book, ask which of these three they mean.
Reading the novel alongside the show
Prose can put the reader close to a character without a performer having to voice the thought. The stage can use music, spacing and movement to suggest the same thing. Choose a scene available in both forms and mark one passage where narration does work you remember being done through timing or choreography.
Finding an edition
Search a library catalogue by author and title together. Match the year and publisher to the copy you actually used, particularly when page numbers matter. Simon & Schuster lists a Shulman edition under ISBN 9781982147150. For archival material, the Library of Congress exhibition includes manuscripts and notes as well as descriptions of how the musical developed.
Scripts and performance
An edition you can borrow or buy serves reading. For a public production, contact the relevant licensing provider and check territorial availability and materials. The MTI catalogue is the starting point for the original stage version.