open book

idiomatic

(idiomatic) Something of which salient aspects are obvious or easily interpreted.

Japanese Meaning

明確で、容易に理解できる事柄（比喩的に、秘密や複雑さがなく、誰にでも分かりやすい性質を持つもの）

Her emotions were an open book during the meeting, so no one was surprised by her reaction.

会議中、彼女の感情は一目瞭然だったので、誰も彼女の反応に驚かなかった。

Sense（1）

(idiomatic) Something of which salient aspects are obvious or easily interpreted.

There is one, with a dark and thoughtful eye,

Who is to all others a mystery;

But his soul is to me an open book,

And I read his mood in his slightest look; ...

Sense（2）

(idiomatic) A person who through naivete responds candidly to questions or openly displays their emotions or intentions.

Sense（3）

(mathematics, topology) An open book decomposition.

Sense（4）

(climbing) An inside angle in the rock.

open books

（ plural ）