someone or something easily understood or interpreted; something very clear:
The child’s face is an open book.
a person or thing without secrecy or concealment that can be easily known or interpreted

pertaining to a test during which a student may refer to a book or books

He did not prepare very much for the open-book test.

something or someone that is easily understandable; something that is very clear; someone who conceals nothing

Her life is an open book.
Something or someone that can be readily examined or understood, as in His entire life is an open book. This metaphoric expression is often expanded to read someone like an open book, meaning “to discern someone’s thoughts or feelings”; variations of this metaphor were used by Shakespeare: “Read o’er the volume of young Paris’ face,” (Romeo and Juliet, 1:3) and “O, like a book of sport thou’lt read me o’er” (Troilus and Cressida, 4:5). [ Mid-1800s ]
For an antonym, see closed book


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