Let me start by saying, I subscribe to the school of thought that fascism is best thought of as an ideology first. I go further than most though, in recognizing that the reason why fascism is so notoriously difficult to nail down, definitionally, is because it is an incoherent worldview, both internally and externally. So, to study fascism is to knowingly engage in a contradiction - trying to apply a rational approach to a system that is
ontologically irrational. You just kind of have to make peace with that fact, otherwise, you might go a little crazy.
Anyway, here is what I recommend, from my own collection,
Ur-Fascism (essay) - Umberto Eco
Strongmen - Ruth Ben-Ghiat
Anatomy of Fascism - Robert Paxton
The Origins of Totalitarianism - Hannah Arendt
The Coming of the Third Reich - Richard J. Evans
And although it's not without its problems, I can't help but recommend The Rise and Fall of the Third Reich. It was the first truly "grownup" book I ever read, and had a huge impact on me. The first 1/4 or so covers a lot of the early history and philosophy of fascism and Nazism.