English Idiom: Read someone like an open book
This is a very popular little idiom, and if you have a significant other or better half in your life, then you will understand what it means precisely. To read someone like an open book means that you know what they are thinking without asking them or them telling you. It is scary sometimes.
My wife and I have been together for nearly 18 years. She can read me like an open book.
She always knows what I am thinking exactly, at all times.
The problem is we have been together for 18 years, and I still have absolutely NO IDEA what she is thinking, ever!
From experience, I can warn all gentlemen that if you have a significant other or just any woman in your life, be careful because she can read you like an open book.
Women are omnipotent!