This is the translation I see in the app, "We are reading one man's books." That makes even less sense then the translation above. This sentence needs correction...over a year? C'mon Duo!!
We are not supposed to be learning genitive that doesn't exist in translation. "We are reading the man's books" is the same and should be accepted - yes I understand that "the man's" would be different in Danish - then use a different sentence!