open book

American  

noun

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

   The child's face is an open book.

---

open book British  

noun

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

---

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.

---

Etymology

Origin of open book

First recorded in 1850–55

Example Sentences

See Examples For:

open book

The open book review of Stormont's finances was conducted after the Executive overspent by £400m last year.

From BBC ● Apr. 21, 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

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 ● Dec. 16, 2025

Despite this bout of caution, Einbinder can also be an open book.

From Los Angeles Times ● Aug. 13, 2025

When a confused dream woke her with a start sometime in the night, Mo was still sitting up in bed, the open book in his hand.

From "Inkheart" by Cornelia Funke