I have a special place in my heart for books which combine the utmost intellectual rigour in their research and method, with the most off the wall subject matter imaginable. Perhaps it is the contrast between form and content, perhaps it is the breadth and depth of the conclusions reached by the microscopic examination of insanity.
Whatever the reason, Pursuit of the Millenium is brilliant example of the kind. I suppose you can tell that a book has reached a certain stature when it is referenced in later classics, and that the book must be especially wonderful of its kind if it attains this status despite pertaining largely to the arcane matter of medieval religious sectarianism. If this is true, then the reference to Cohn's opus in On Chesil Beech by Ian McEwan (in which principle male character is reading the book) is some sort of validation - especially given McEwan's predilection for using his novels to drop unsubtle hints about the sort of activities he considers culturally worthwhile.
The book is a succession of remarkable stories, interlaced with the development of the ideas which informed each instance of revolutionary eschatology. Similar motifs and patterns crop up again and again with such surprising reguality over periods of centuries that it is hard not to think that the commonalities must point to some sort of underlying human or structural bias. What it is though, is hard to say. Because it deals with revolutionary movements in the dark ages, it is also a fascinating comparative text for anyone interested in the revolutionary and social movements of the recent past - though Cohn does arguably lay that on a little thick at times.
No real prior knowledge of the subject is necessary to read the book, though I personally needed to make occasional recourse to Wikipedia to remind me of what some theological terms mean.
This, in case you haven't noticed, is a glowing review. Read this book!