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

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

He's noticed a marked improvement this year, where at university some of his exams have been open book, which he says he has preferred to his assessments at school.

From BBC • Aug. 19, 2025

The Marines went to extreme lengths to shield Sasaki from the media in his five years with them, so much so that the intensely private Ohtani seems like an open book by comparison.

From Los Angeles Times • Dec. 11, 2024

The probation department will also examine Trump's background, which is pretty much an open book.

From Salon • Jun. 6, 2024

He nodded at the open book beside him—Janson’s History of Art, which was cut practically to tatters.

From "The Secret History" by Donna Tartt