Historiography of Hitler
Historians have taken many approaches to wriing about Adolf Hitler and explaining this perhaps unexplainable man: the historiography of Hitler. A great many works have tried to explain Hitler, with varying perspectives and conclusions. Indeed, Ron Rosenbaum analyzed twenty views, by recognized scholars, in his book Explaining Hitler; no two reach the same conclusion. New academica approaches developed in the 1970s, and have accelerated, especially with the availability of newly declassified documents in the 1990s.
Beginning with Konrad Heiden, biographers began studying Hitler during his life. His book, published in 1936, ended in the summer of 1934. After his death, an early and specialized work was Hugh Trevor-Roper's The Last Days of Hitler (1947).
Perhaps the first major biography was Alan Bullock's 1952 Hitler: A Study in Tyranny. That was among the first to deal with Hitler's sexuality, although there had been a classified wartime study of his psyche, with substantial attention to sexuality, done by William Langer for the U.S. Office of Strategic Services. Bullock, to some extent, revised some observations, but not their core, in a 1991 biography Hitler and Stalin: Parallel Lives. While a general history of Nazi Germany, William Shirer's 1960 Rise and Fall of the Third Reich provides much information. It discusses both Hitler and his close associates. 
In the 1970s, according to Ron Rosenbaum, there were two major schools of thought in Hitler scholarship. The psychohistorical assumed Hit;er actions were due to mental illness, and could be analyzed with the tools of abnormal psychology. In this period, the other school was "ideological", which would later be the core of the "intentionalist" school. Rosenbaum mentions Robert Waite's book, The Psychopathic God: Adolf Hitler, as an example of the psychohistorical approach.
There were at least two earlier psychological studies. Langer's OSS study, of course, was psychologically based. Another source is G.M. Gilbert's The Psychology of Dictatorship, not specifically a history of Hitler alone, but having the insight gained from directly interviewing and giving psychological tests to the Major War Criminals at Nuremberg.
Functionalists versus intentionalists
Lothar Machtan, in the Hidden Hitler, which focuses on Hitler's sexuality and relationships, presents a model of Hitler historiography.  Also starting in the 1970s, but especially a decade or two later, there were two main schools of Hitler biography: functionalists and intentionalists; the latter are also called structuralists. Functionalists saw Hitler as motivated by the exercise of power regardless of purpose, while the intentionalists focused on his specific vision. A transitional work in this direction, among the first published in German, was Joachim Fest's 1972 book, Hitler.
According to Machtan, Hans Mommsen was the leader of the functionalists. He considered Hitler a "political counterfeiter" who succeeded because he was constantly overrated, extremely effective with propaganda but not in performance.  Machtan considers Ian Kershaw to be the leading current historian, attempting to unify the two schools.
A more recent aspect of the functionalist versus intentionalist debate is described by Ian Kershaw in a 2008 book: did Hitler actually issue an explicit, if oral, order for the Final Solution? For many years, this had been assumed: that he gave verbal instructions to Hermann Goering and Heinrich Himmler, who conveyed it to Reinhard Heydrich. While Kershaw has said, in interviews, that had there been no Hitler, there would have been no Holocaust], he increasingly doubts there was a direct order. By no means is this universally held, and none of the significant historians doubt Hitler intended the effects of the Holocaust. Rather, the argument is that the extremely non-bureaucratic Hitler might have set conditions, but never directed.  Yehuda Bauer calls the Final Solution the result of a "stage by stage development in 1941," and, with respect to the two schools, explains the functionalist view as assuming that central ideology and decisions were less important than had previously been thought, but they agree that "without approval by Hitler and his inner circle, the murder would have been impossible."  He wrote that the Holocaust specifically was a matter of ideology, and intentionalism versus functionalism were outdated ideas.
- Ron Rosenbaum (1998), Explaining Hitler: the Search for Origins of His Evil, Random House, ISBN 0-679-43151-9
- Konrad Heiden (1999), The Fuhrer: Hitler's Rise to Power, Basic Books (reprint), ISBN 978-0786706839
- John Lukacs, "The Hitler of History", New York Times
- Walter C. Langer (1943), A Psychological Analysis of Adolph Hitler: His Life and Legend, Office of Strategic Services
- William Shirer (1960), Rise and Fall of the Third Reich, Simon & Schuster
- Rosenbaum, 288-289
- G.M. Gilbert (1950), The Psychology of Dictatorship: based on an examination of the leaders of Nazi Germany, Ronald Press Company
- Lothar Machtan (2001), The Hidden Hitler, Basic Books, pp. 1-15
- Joachim Fest (1973), Hitler, Harcourt Brace Jovanovich, pp. 74-75
- Hans Mommsen. Hitlers Stellung im nationalsoczialistenschen Herrschaftsystem, p. 144, quoted by Machtan, p. 12
- Ian Kershaw (2008), Hitler, the Germans and the Final Solution, Yale University Press & Yad Vashem, ISBN 978-0-300-12427-9, pp. 93-101
- Yehuda Bauer (2002), Rethinking the Holocaust, Yale University Press, p. 176
- Bauer, p. 4-7