an open book

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an open book

1. Something that is easy to understand or decipher. These legal contracts are anything but an open book. Don't worry, this recipe is an open book, even if you're not an experienced baker. Modernist poems have so many allusions to other works that they're not exactly an open book
2. One who acts (or purports to act) honestly, with no secrets. Ask me anything, I'm an open book. Adam says he's an open book, but I've seen him sneaking out of his house late at night. If he says he's an open book, then he's lying—I know he works for the mob!
See also: an, book, open
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.

(like an) open book

Fig. [of someone or something] easy to understand. Jane's an open book. I always know what she is going to do next. The committee's intentions are an open book. They want to save money.
See also: book, open
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

open book

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.
See also: book, open
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

an open book

If a person's life or character is an open book, nothing about it is kept secret. `Their lives are an open book,' says a spokesman. `They are good people and she has always been a good kid.' His life is an open book. You know which girl he's dating, which car he's driving. Compare with a closed book.
See also: an, book, open
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

an ˌopen ˈbook

a person whose behaviour, attitudes, thoughts, etc. are very easy to understand, either because you know them very well or because they are very open and honest: After living with her for 20 years, she’s an open book to me.His life is an open book. He has no secrets. OPPOSITE: a closed book (to somebody)
See also: an, book, open
Farlex Partner Idioms Dictionary © Farlex 2017
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References in classic literature ?
The familiar details came out: the stag's horns, the bookshelves, the looking-glass, the stove with its ventilator, which had long wanted mending, his father's sofa, a large table, on the table an open book, a broken ash tray, a manuscript book with his handwriting.
Enamoured of the South, of Provence, of its people, its life, its sunshine and its poetry, narrow-chested, tall and short- sighted, he strode along the streets and the lanes, his long feet projecting far in advance of his body, and his white nose and gingery moustache buried in an open book: for he had the habit of reading as he walked.
Next morning when the valet came into the room with his coffee, Pierre was lying asleep on the ottoman with an open book in his hand.
"We are lost!" was written as plainly upon Taylor's face as though his features were the printed words upon an open book. He was thinking of the launch, and of the launch alone.
Still she was not astonished when, as she was partaking of a modest dinner late in the afternoon, looking into an open book, stroking the cat, which had made friends with her--she was not greatly astonished to see Robert come in at the tall garden gate.
"A learned man in a cynical and torn dress holding an open book in his hand."
It was Tars Tarkas, and I could read his thoughts as they were an open book for the undisguised loathing upon his face.
"By those who, at Havre, had, with infernal perspicacity, read my heart like an open book."
To him, the trail of the raiders would be as plain as the printed page of an open book to her.
The white-headed boy then put an open book, astonishingly dog's-eared upon his knees, and thrusting his hands into his pockets began counting the marbles with which they were filled; displaying in the expression of his face a remarkable capacity of totally abstracting his mind from the spelling on which his eyes were fixed.
Instead of looking up at us in her usual straightforward way, she sat close at the table, and kept her eyes fixed obstinately on an open book.
No longer was there a single jungle spoor but was an open book to the keen eyes of the lad, and those other indefinite spoor that elude the senses of civilized man and are only partially appreciable to his savage cousin came to be familiar friends of the eager boy.
The day was damp, and they were not going to walk out, so they both went up to their sitting-room; and there Celia observed that Dorothea, instead of settling down with her usual diligent interest to some occupation, simply leaned her elbow on an open book and looked out of the window at the great cedar silvered with the damp.
On the surface of the ground or through the swaying branches of the trees the spoor of man or beast was an open book to the ape-man, but even his acute senses were baffled by the spoorless trail of the airship.
The boy was sitting with his elbows on the table, and his head leaning on his hands, and before him an open book, on which his tears were falling fast.