open book

Noun

open book (plural open books)

1. (idiomatic) Something whose salient aspects are obvious or easily interpreted.
   • 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
2. (idiomatic) A person who responds candidly to questions or openly displays their emotions or intentions.
   • 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
   • 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

   Synonyms:

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

3. An open book decomposition.
4. An inside angle in the rock.