"You can close your books now."
Translation:I kan lukke jeres bøger nu.
I am not sure, but I am thinking maybe it would be weird to tell someone to close his/her books, unless you're a teacher in a class...
I think because generally someone would only have one book open at once, so since the books are plural, it is meant to refer to multiple people. (Of course it would be possible to have multiple books open at once, it just doesn't happen to be the intended meaning of this sentence.)