Read somebody like a book
Definition 1 of 1
Pro Tip 1/3Intimate Insight
Use when you know someone's thoughts or feelings without them saying anything. 
Whenever he's troubled, I read him like a book and know exactly how to help.
Pro Tip 2/3Observation Skills
Implies you've watched someone closely and understand their habits or quirks. 
After working together for years, I can read her like a book.
Pro Tip 3/3Not Literal
It's a metaphor suggesting people can be understood as easily as reading text from a book. 
He's got a poker face, but I can still read him like a book.