This really is an excellent question. Quoting Michael Parenti’s Blackshirts and Reds, page 2:

Among the thousands of titles that deal with fascism, there are a few worthwhile exceptions that do not evade questions of political economy and class power, for instance: Gaetano Salvemini, Under the Ax of Fascism (New York: Howard Fertig, 1969); Daniel Guerin, Fascism and Big Business (New York: Monad Press/Pathfinder Press, 1973); James Pool and Suzanne Pool, Who Financed Hitler (New York: Dial Press, 1978); Palmiro Togliatti, Lectures on Fascism (New York: International Publishers, 1976); Franz Neumann, Behemoth (New York: Oxford University Press, 1944); R. Palme Dutt, Fascism and Social Revolution (New York: International Publisher, 1935).

These works could sadly use a little updating (Who Financed Hitler, for example, mistakenly cites I Paid Hitler, not realising that it was a literary forgery), but I am less familiar with the latest publications, such as The Third Reich: A History of Nazi Germany and A Companion to Nazi Germany. I cannot attest to their quality.

You have no doubt heard of William Shirer’s famous The Rise and Fall of the Third Reich, and while it certainly isn’t abysmal in quality, the editorialization coupled with the nonsocialist author’s unscientific perspective makes it a low priority. I cite it occasionally; reading it won’t kill you. But there is no rush to read it when there are better works available.