open book

American  

noun

1. someone or something easily understood or interpreted; something very clear.

   The child's face is an open book.

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open book British  

noun

1. a person or thing without secrecy or concealment that can be easily known or interpreted

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open book Idioms  

1. 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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Etymology

Origin of open book

First recorded in 1850–55

Example Sentences

She’s an open book in interviews — she’s an open book everywhere.

From Los Angeles Times

Mears said it works on an open book basis with the Home Office and, following an independent audit, had made agreed repayments to the government.

From BBC

"Now I'm an open book...six years ago, I couldn't even say the words periods without flushing red," she added.

From BBC

Some of Demoff's closest friends in the industry are at the San Francisco 49ers, who now own Leeds United and Rangers too, but "we're all competitors. There's only so much people are willing to share, whereas here, there is no limit. Everybody realises that being an open book makes us all better".

From BBC

Eilish has been “somewhat of an open book” as long as she can remember.

From The Wall Street Journal