DL presented me with a different option - "We are the books authors" no possessive apostrophe. It won't take feedback that the english is not right. I'm English and expect a possessive apostrophe after the s for a plural ending in s, apart from its. Am i wrong?
I'm not sure if this is related, but I know that Duolingo has a bug in which the possessive apostrophe of plural nouns is treated as a typo.
COuld be connected - though it accepted me putting in the possessive apostrophe, it was only its example that lacked it - which to me is a 'school boy error'!
Genitive in English has an apostrophe 's for singular nouns and 's for plural if it comes before a noun. So if you write it like this 'We are the books' authors' it is correct not the underlined version given in the prompt. Of course you can write the sentence as above when it would be incorrect to have an apostrophe hence 'we are the authors of the books'.