>>24529192 (OP)Blackshirts and reds by Parenti. Then look up marxists who wrote about fascism. Your best bet is those who experienced fascism, but also lived long enough to have full historical context of the end of the biggest European Fascist movements.
Probably books from around the 50s or 60s will be the best source.
Contemporary scholars can also be good, but it is more variable, because many have never experienced fascism. This is a problem because fascism is a "fashionable" topic. Thus grifters who have no clue to also write about fascism just to make a buck.