I've been reading The Rise and Fall of the Third Reich by William L Shirer. If you ever want a comprehensive account of how the Nazis came to power, this 1300 page monster is it. I feel like if I don't pay attention for one sentence I miss a vital detail to the story. It's truly fascinating and horrifying at the same time.
If you want a soul-crushingly depressing and historically fascinating follow-up, read Bloodlands: Europe Between Hitler and Stalin by Timothy Snyder.