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 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

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

From Los Angeles Times ● Dec. 17, 2025

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

"I see myself as an open book, and I see myself as someone who's been through quite a journey."

From BBC ● Jun. 17, 2025

The tea stops before it splashes onto an open book as though there is an invisible wall surrounding it.

From "The Night Circus" by Erin Morgenstern