How is “an open book” used?

In The Lighthouse (2019), one line is: “You're an open book. A picture, says I.”

Which movie is “an open book” from?

The clip at the top of this page is from The Lighthouse (2019).

if you describe sb or their life as an open book, you mean that you can easily understand them and know everything about them

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if you describe sb or their life as an open book, you mean that you can easily understand them and know everything about them
You're an open book. A picture, says I.
I'm an open book.

Definiciones del Oxford Advanced Learner's Dictionary, 9.ª edición

You're an open book. A picture, says I.

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