Throughout William Gibson's Spook Country Milgrim is reading a book he found in the pocket of the coat he stole. Its a 1961 history of messianic medieval history.
Has anyone managed to identify the book?
I tweeted William Gibson @GreatDismal, including a link to this page. He has just answered "that one" (i.e. the one mentioned here above).
@GreatDismal What book is Milgrim reading in William Gibson's Spook Country?
So it is indeed The Pursuit of the Millennium: Revolutionary Messianism in Medieval and Reformation Europe and its Bearing on Modern Totalitarian Movements by Norman Cohn, as found by sjl.
Ctrl-F
for '1961' found The Pursuit of the Millennium: Revolutionary Messianism in Medieval and Reformation Europe and its Bearing on Modern Totalitarian Movements by Norman Cohn.