"Επιστρέφουν τα βιβλία πίσω."

Translation: They are returning the books.

[gpbalis]

when using the verb returning in this context the word back is superfluous. It isn't used in English. either giving the books back or returning the books but not returning the books back

[Theo_Matrakas]

You're right!!! We deleted this sentence and we replaced it with a new sentence without πίσω.

[Hafizen]

This sentence is still in the new tree without change. As mentioned above, πίσω is superfluous. I reported it as my answer should not be accepted.