Adolf Hitler by John Toland - legendary biographer, and quintessential biography. I listened to the audiobook version, which I think makes it inherently accessible as it is a 44 hour long listen so it is a book I probably would have never actually finished if reading. I found the section around the early 1930's to be the most interesting personally as it follows his meteoric rise in politics. The nuance of someone's "present" (if you were a German in 1933 let's say) success and adoration compared with the devastating legacy we all know now is terrifying and infinitely interesting.

From the Holocaust section of the booklist, the first entry Hitler: A Biography looks like it might be useful.