What does "open book" mean? Is it a common expression?

Native speaker’s answer
Rebecca
"Open book" is an expression that means someone is very honest about their life that their life or stories are easy to learn about and to understand. Ex: My aunt is an open book. She tells me about all of the things she has done wrong and how she learned from them. Ex: I'm an open book. It's much easier than trying to hide who I am.