open books

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
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

   Synonyms:

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

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