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Discover LudwigThe phrase "open book" is correct and commonly used in written English.
It means that something or someone is transparent and easy to understand or interpret. Example: "Her emotions were like an open book, easily readable on her face."
Dictionary
open book
noun
Something of which salient aspects are obvious or easily interpreted.
Exact(59)
Iago's an open book.
You open book.
The night is an open book.
Our life is an open book".
It was an open book".
"Open Book," by Scott Anderson.
I'm a very open book.
Cambridge : Open Book Publishers, 2017.
The tests are open book.
Ms Yellen's views are an open book.
Similar(1)
– It's an open-book test.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com