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