To be fair, it wasn't Mein Kampf that caused Hitler rise to power. It is widely agreed that from a philosophical and literary viewpoint, it is a laughable load of old drivel. Would normally have sunk without trace but for the fact of Hitler's notoriety. Hitler's power lay in the fact he was the consumate politician, worked and worked and worked on his oratory skills, plus the economical position of galloping inflation. Luck, chance, advantage.
(Writing books and spouting nonsense has never been censored in the UK, although there was a 'Section 28' ban on gay writings at one time - and even DH Lawrence was 'banned' (Lady Chatterley's Lover) - but not because of 'free speech' more to do with moral issues of the day.)