1. off book - Adjective
2. off book - Phrase
(theater) No longer needing the script to rehearse.
After 5 days studying the script, Jane is now off book, and can concentrate on gesture more.
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(theater) Able to perform scripted material from memory, without need to refer to the text.
(accounting) Off the books, not officially recorded.
Source: en.wiktionary.orgoff-book
Part of this style of training includes actors memorizing lines to be able to work off-book, a term that means being able to work without a script. Source: Internet
These discoveries are made with the help of former MI5 officer Victor Oxley, "Bedrock" - an off-book MI5 asset who operated as an assassin during the Cold War in similar fashion to Talanov, aka Zenith. Source: Internet