What is the origin of the idiom "open book" and when to use it?

The origin of the idiom "open book" can be traced back to the literal act of opening a book and being able to read its contents. The phrase metaphorically suggests that a person is like an open book, readily sharing information about themselves without reservation. While the exact origin of the idiom is unclear, it likely emerged from the idea that a book, when open, reveals its contents and allows others to gain knowledge or understanding. The phrase encapsulates the concept of transparency, honesty, and approachability, emphasizing the willingness of an individual to openly share their thoughts, emotions, or personal experiences without concealing or withholding information. It is commonly used to describe someone who readily expresses their thoughts, feelings, or personal experiences without hesitation.