Sebastian Haffner manages to summarize Hitler's career in nearly 150 pages, giving his personal view of his successes, failures, crimes and obsessions. Though more a historical essay with plenty of personal opinions by the author (a very shrewd observer on the other hand) than a regular biography or a serious history book, the volume is worth reading as a summary of Hitler's main traits and a thought-provoking book.
Personally, I liked the author's "The German Revolution" better, but his keen analytical powers are present in this short volume as well, though his style is more personal and so more open to criticism.