I think Mein Kampf was influential because writting it expressed Hitler's thoughts and the thoughts expressed in it influenced the world.
Also, I would place the Avesta much earlier than 500 B.C. - perhaps 1000 B.C. or even earlier, according to linguistics experts who studied the language it was written in.
Hitler would have still come to power even if he never wrote MK. That's the issue. The book itself didn't influence people in a meaningful way until after the fact.