Open Book Meaning

(climbing) An inside angle in the rock.

Example: 1860, Abby Maria Hemenway, The Priest and the Maiden in Poets and Poetry of Vermont
  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; ...
1898, John Wesley Powell, Truth and Error or the Science of Intellection,
  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.
1921, Edgar Rice Burroughs, Tarzan the Untamed,
  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.
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.
1986, Adrienne von Speyr and Hans Urs von Balthasar, The Christian State of Life,
  He is an open book. Everyone can see that he removes nothing, conceals nothing, reserves nothing for himself.
1997, Matthew J. DeLuca, Best Answers to the 201 Most Frequently Asked Interview Questions,
  Chapter 3: Are You an Open Book? How Much Do I Need to Disclose?
2002, Linda Lange, Feminist Interpretations of Jean-Jacques Rousseau,
  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."