History is generally written by the victor. There are though, tons of records of the regime, including Hitler order, I'd assume, in archives, ad the Nazis were impeccable record keepers. In fact, much of those records were used in the war crimes trials after the war. What led him is the million dollar question; he was surely a personality disordered person, likely a narcissist. He did, for a time, bring prosperity to Germany, but his biggest commercial success is probably the thousands of books taking up your very questions.