What does 'book' refer to in a theatrical context?

In a theatrical context, 'book' refers specifically to the prompt book or the script that contains the dialogue, stage directions, and other essential elements of a musical. This document serves as a crucial resource for the director, stage manager, and performers as it outlines what needs to be said and done on stage. The prompt book usually includes cues for lighting, sound, and scene changes, making it an essential tool for coordinating the various aspects of a production and ensuring that everyone involved has a comprehensive reference for their performances.

While collections of play scripts may be referred to as a 'book' in a broader sense, the specific use of 'book' in relation to a musical points directly to the detailed script that guides the production. This usage highlights the careful orchestration of elements that is unique to musical theater, where the integration of music, lyrics, and dialogue requires meticulous attention to detail found within the prompt book. The other options, such as diaries or rehearsal notes, do not encompass this specific functionality or significance attributed to a theatrical 'book.'