Well I have to agree that "return" is the first word that comes to mind, and one thinks of returning books to a library, but in this day and age, perhaps sending books back to Amazon happens a lot more often. :)

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.