I can't pretend to have all the historians named Norman Coh(e)n sorted out,
but here are a few as their books are listed on amazon.com.
Norman Cohn, The Pursuit of the Millennium (1990).
Norman Rufus Colin Cohn, Warrant for Genocide.
Norman J. Cohen, Voices from Genesis.
Norman M. Cohen, Jewish Bible Personages in the New Testament.
Norman S. Cohen, Civil Strife in America.
I think you're right that the first two books are both by RC (no e), and
thanks for correcting me on that item. But who are J, M, and S? I found a
photo of RC, who has white hair and a full beard. The Norman Cohen I heard
speak (at a medieval conference) had dark hair, no beard, and was between 45
and 55 years old.