I am reading William Shirer's rise and fall of the third reich. Over all the book has been tremendously informitive. The majority of the sources in it are derived from the nazi archives seized after the war. And while that may give great insight into the internal workings amd history of the Nazi regime, I don't know if it will provide the most accurate picture of the war in the East. I ask this because I have heard it said we (America/the west) did not have a very complete or accurate picture of this part of the war until the soviet archives were opend up after 1991. Given that the book was written in the 60s this means the author had no access to these records. So is it an inaccurate picture of the war between the USSR and Nazi Germany?

tldr: is a book primarily composed of Nazi recordes without access to a soviet history accurate?