Translation:Instead of one book, he writes two.
I translated "he writes US two", because of the "NE scrive". Wrong. Why? What's the use of NE?
cristisalceanu - correct me if i'm wrong but i believe "ne" takes the place of the word "books". instead of saying " invece di un libro, lui scrive due libri" which sounds repetitious you use lui ne scrive due, the ne goes back to the books in the other sentence. It is understood to stand for the 'books", in this case.
Something like that :) More about it here: http://blogs.transparent.com/italian/tricky-little-words-%E2%80%9Cne%E2%80%9D/
That link didn't resolve for me. This one did:
It doesn't quite work like that. When you are talking about "one", like in "the number one", eg "there is one", or "one, two, three, ...", yes, you always use "uno" (well, I think you can use either "un" or "uno", but I'm not sure if there's a rule)
Not so with "one of something", like in "one horse" or "one book". In Italian you'd write "un libro" or "un cavallo" both to say "a book"/"a horse" (used as an article) and "one book"/"one horse" (used as a numeral adjective). Only for masculine singular nouns, mind you. This is common with other romance languages like French or Spanish.
But you still say "uno something" if that something starts with impure "s"