What's wrong with "Perhaps you know the books"? "All" is definitely optional in English second person plural. Reported.
Yes. Optional, and possibly even wrong. If you really were translating a Latin document and weren't sure whether it meant "you all" or "you both" I think you'd just leave it as "you".
Also, you can address a group of people in the plural (in a language that has 2ndPP) but if you add the word "all" it really means literally every single one of you. There's a slightly different twist.
This was fixed seven days ago. But the software sometimes takes a long time to update changes to users :(
Why it is southern Latin? I don't know about the topic, neither a native english speaker.