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

"He is an open book and gives long answers at media conferences - what you see is what you get."

From BBC • Jun. 12, 2026

The back features a genie’s lamp resting on an open book and a feathered quill with an olive branch in the background—symbols of peace, knowledge and learning.

From The Wall Street Journal • Feb. 4, 2026

"We're a very open book in this family," she said.

From BBC • Sep. 2, 2025

“I am an open book in that I truly want to connect with people,” she says.

From Los Angeles Times • Jun. 26, 2025

He massaged some life back into them and limped heavily to the table where the septon was snoring softly, his head pillowed on an open book in front of him.

From "A Game of Thrones" by George R.R. Martin