This is an English phrase that is used to describe a person who is very open and not hiding anything. It can be used to describe a person or something about a person. For instance: “He is like an open book.” Another possibility is: “His personal affairs are like an open book.”

This kind of person would likely not hide their diary and would let friends read it. A variation of the “open book” concept is used when a person is not saying something, but a person can still “read” what they are not saying in other ways.

For Example:

“I could read her true feelings like an open book, so it was easy to know how much she loved me.”