Those of you inclined to a dim view of cultural historians as a class will note that Cohn started out his scholarly career as a linguist.
I've read all his main works except for Noah's Flood, and none of them are dispensable, although they so completely bury the needle in their area that virtually all serious books written after Cohn simply begin from the ground he so laboriously cleared. Hence, it's hard for modern readers to see what all the hubbub was about 50 years ago.
That said, there's even more in his study of apocalyptic religion, The Pursuit of the Millennium: Revolutionary Millenarians and Mystical Anarchists of the Middle Ages, than its main takeaway point -- that Nazism and Communism are simply reiterations of a cultural pattern that goes back to before the Anabaptists of Munster, Joachim of Fiore, and the Beggar's Crusade. In the course of the work, Cohn also finds the "Final Emperor" trope of great import. (We call him "King Arthur" around these parts, but there's lots more to it than him, and it's Cohn who identified the first one, Constans I.)
Europe's Inner Demons is less essential, as its primary task -- the clearing out of nonsense about witchcraft -- has been completed to the satisfaction of historians everywhere by Briggs' Witches and Neighbors. But Cohn's depiction of the "template of persecution" is unbeatable for clarity and strength, and robustly complements his other works. Cohn's study of apocalypse and dualism, Cosmos, Chaos, and the World to Come, is likewise excellent, and was one of the first books to begin the amazing recovery of the Persian influence on Western society that 2,000 years of (justifiable) Herodotos worship had managed to obscure. But again, Stoyanov's The Other God has more than satisfactorily expanded on Cohn's initial work. Contrariwise, nobody has yet written a better book on the contemptible Protocols of the Elders of Zion forgery than Cohn's Warrant for Genocide, so if you're curious, this is the book to check out.
Rigorously accurate, impeccably literate, incisively imaginative, and capable of simultaneous synthesis and analysis, they don't make them like Norman Cohn very often.