Who wrote Goldstein's book? Is what the book says true?

From the text:

You have read THE BOOK, Goldstein’s book, or parts of it, at least. Did it tell you anything that you did not know already?’
‘You have read it?’ said Winston.
‘I wrote it. That is to say, I collaborated in writing it. No book is produced individually, as you know.’
‘Is it true, what it says?’
‘As description, yes. The programme it sets forth is nonsense.