someone or something easily understood or interpreted; something very clear:
The child's face is an open book.
Origin
1850-55
Examples from the web for open book
When you are involved in politics, your life is an open book, and people can come in who don't necessarily have good intent.
If to the swallow life is an open book, to the snail it is unfathomable.
The exams are open book and the required books for each exam category can be found on the licensing requirements for each license.
Successful completion of an open book written jurisprudence exam.
British Dictionary definitions for open book
open book
noun
1.
a person or thing without secrecy or concealment that can be easily known or interpreted
Contemporary definitions for open book
noun
something or someone that is easily understandable; something that is very clear; someone who conceals nothing
Examples
Her life is an open book.
Idioms and Phrases with open book
open book
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 ]