"Read someone like a book" is an idiom that refers to the ability to understand and interpret someone's thoughts, feelings, intentions, or behavior easily, often without them speaking up. Some synonyms for this phrase include "know someone like the back of one's hand," "understand someone inside out," "see through someone," "decode someone's body language," "read someone's mind," and "have a sixth sense about someone." These expressions suggest that the person has a deep knowledge of the other's personality, history, habits, and emotions, and can predict their reactions or motivations accurately. They imply a perceptive, insightful, or intuitive aptitude that can be useful in various scenarios, from social interactions to professional negotiations.