Kind of like the old metaphor of hedgehogs and foxes, there are historians who have worked in one particular area, and ones who have tried to range widely. Trevor-Roper was one of the latter. He was good at scornful critiques and feuds: the one which I first read about as a student was the debate in the 1940's-50's over the role of the gentry in the causes of the English Civil War R.H. Tawney, Lawrence Stone and others had made a case that a rising Puritan gentry had come into conflict with a declining Anglican aristocracy, something that seemed rather intuitive ( that nobility would be Anglican, gentry Puritan, gentry able to make more money from rents and farming than nobility). Tawney was a very important figure, but Trevor-Roper popped up and attacked this- and them- , saying the reverse was happening, that nobles were doing well, gentry were falling. Eventually everyone became very tired of the debate, when it became apparent ( after computers gave better data analysis) that really everyone was doing everything- falling Anglican gentry, rising Puritan aristocrats, etc. and there were no clear conclusions to be drawn. So, Tawney hadn't been right. But neither was Trevor-Roper.

He was good at critiquing others' big books ( like Arnold Toynbee's over-grand Study of History). And he was pretty fiercely conservative, at a time when probably most English scholars ( like Christopher Hill, Eric Hobsbawm) were not, so a good foil for overly-contented Marxists. He once made a comment that, while it used to be historians were standing on the shoulders of giants, they were now reduced to standing on molehills, jealously guarding them from other scholars. But it's significant that he himself never really had his own molehill, never put out a big solid book ,patiently assembled, with a lot of research and a lot of sources mastered. So, you can't really recommend somebody start with one of his books if they want to get into a subject. ( I loved The Hermit of Peking, but I would never dare to use it alone as a source late 19th c.-early 20th c. China. ) And one of the things about having mastered your molehill is that you aren't so easily fooled . You could say he just didn't know enough about Germany and the Nazis, and so was too easily misled about those Hitler diaries- the thing for which, sadly, he's probably most famous. Still, his last two books: The Invention of Scotland: Myth and History  and Europe’s Physician: The Various Life of Sir Theodore de Mayerne  got very good reviews when they came out after his death, are said to be more solid and less flash and dash.

Hugh Ascherson had a nice article on a biography of Trevor-Roper in the August 19 2010 London Review of Books that summarizes his story pretty well.