know like a book

To thoroughly and completely understand someone's emotions, motivations, etc., or how something works. After 30 years of marriage, my wife and I know each other like a book. Let's ask Jeff—he knows the security system like a book.

Also, know like the back of one's hand or know backwards and forwards. Be extremely familiar with or knowledgeable about; understand perfectly. For example, I know Greg like a book-I'm sure he'll come, or I know this town like the back of my hand, or John knew his part backwards and forwards. The first of these hyperbolic idioms, dating from the early 1800s, has a close cousin in read like a book, which means "to discern someone's intent," as in I can read Greg like a book; also see under open book. The second ( back of hand) dates only from the mid-1900s. Also see backwards and forwards, def. 2; inside out, def. 2; know all the answers.

To be very familiar or knowledgeable about something or someone. Presumably this expression, which dates from the early nineteenth century, alludes to a book one has read very carefully. Also see read someone like a book. A much newer synonym is to know like the back of one’s hand, where the allusion is obvious; it dates from the mid-1900s.