open book

plural open books

1. idiomaticSomething whose salient aspects are obvious or easily interpreted.

   2009, May 21, Barack Obama, speech, Protecting Our Security and Our Values But I have never argued – and never will – that our most sensitive national security matters should be an open book.

   3 more examples

   There is one, with a dark and thoughtful eye, Who is to all others a mystery; But his soul is to me an open book, And I read his mood in his slightest look; ...

   1860, Abby Maria Hemenway, “The Priest and The Maiden”, in Poets and Poetry of Vermont

   This is the secret of Hegel, and the key to his hieroglyphics, and, if consistently used to interpret the sayings of his logic, it becomes an open book.

   1898, John Wesley Powell, Truth and Error or the Science of Intellection

   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.

   1921, Edgar Rice Burroughs, Tarzan the Untamed

2. idiomaticA person who responds candidly to questions or openly displays their emotions or intentions.

   Chapter 3: Are You an Open Book? How Much Do I Need to Disclose?

   1997, Matthew J. DeLuca, Best Answers to the 201 Most Frequently Asked Interview Questions

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   He is an open book. Everyone can see that he removes nothing, conceals nothing, reserves nothing for himself.

   1986, Adrienne von Speyr, Hans Urs von Balthasar, The Christian State of Life

   Emile's most striking characteristic is that he hides nothing. He is an open book, acting the same in private as in public — "Emile is worse at disguising his feelings than any man in the world."

   2002, Linda Lange, Feminist Interpretations of Jean-Jacques Rousseau

   Synonyms: one who wears one's heart on one's sleeve

3. mathematicsAn open book decomposition.
4. climbingAn inside angle in the rock.