Translation:We sent back the books
Either We sent the books back or We sent back the books is correct. (I'm assuming "backs" was a typo because that is definitely not correct.)
I slightly prefer to put "back" after the object, and this is necessary with a pronoun (eg. "We sent them back" is correct and "
We sent back them" is not), but "We sent back the books" is absolutely correct and quite normal English.