An open book Definition 1 of 2Pro Tip 1/3Predictable Personality Refers to someone whose thoughts and actions are easy to guess. As soon as he heard the news, I knew he would offer to help; he's an open book. Pro Tip 2/3No Secrets Describes someone who doesn’t hide their feelings or secrets. You can ask her anything, she’s an open book. Pro Tip 3/3Emotional Transparency Indicates that a person's emotions are easy to read and understand. After the match, his disappointment was an open book.