be an open book

1. To be easy to understand or decipher. I wish these legal contracts were an open book. Don't worry, this recipe is an open book, even if you're not an experienced baker. Modernist poems have so many allusions to other works that they're not exactly an open book.

2. To act (or purport to act) honestly, with no secrets. Ask me anything, I'm an open book. Adam says he's an open book, but I've seen him sneaking out of his house late at night. If he says he's an open book, then he's lying—I know he works for the mob!

an open book, he/she is (like an)

Very obvious. See read someone like an open book.