Read somebody like a book - Idioms by The Free Dictionary
read somebody like a book
read (one) like a book
To easily analyze and thoroughly understand one, especially one's emotions, motivations, etc. You won't be a very successful thief with that nervous demeanor—the cops will read you like a book. I can read you like a book, and I can tell that something's wrong, so please tell me what it is.
ˌread somebody like a ˈbook (informal) understand somebody so well that you can guess what they will say or do before they say or do it: She found that after living with him for a year or more, she could read him like a book.