He is an open book, so you always know his thoughts.
open book
idiomSomething whose salient aspects are obvious or easily interpreted.
Because she is such an open book, her friends can tell immediately when something is bothering her.
Despite his attempts to maintain a stoic facade, his expressive face made him an open book to anyone who cared to observe him closely.
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blot one's copy book
To damage one's own reputation through bad behavior.
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book burning
Any mass destruction, removal, or recalling of books, motion pictures, broadcasts, or electronic works as a form of censorship, especially motivated by religious or political objections to the material.
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book dumping
The practice of donating old used books that burden rather than assist communities.
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book in
To check in (to a hotel).
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call someone every name in the book
To call someone many abusive names.
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close the book on
To stop worrying about or trying to do something about; to ignore.
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closed book
A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling.
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for the book
Notable, remarkable or surprising.