open book

Fig. [of someone or something] easy to understand. Jane's an open book. I always know what she is going to do next. The committee's intentions are an open book. They want to save money.

Something or someone that can be readily examined or understood, as in His entire life is an open book. This metaphoric expression is often expanded to read someone like an open book, meaning "to discern someone's thoughts or feelings"; variations of this metaphor were used by Shakespeare: "Read o'er the volume of young Paris' face," ( Romeo and Juliet, 1:3) and "O, like a book of sport thou'lt read me o'er" ( Troilus and Cressida, 4:5). [Mid-1800s] For an antonym, see closed book.