Great article. It draws heavily on material cited in John J. O’Neill’s eye-opening book “Holy Warriors.” I highly recommend it.
The book for everyone else
Mohammed and Charlemagne Revisited: The History of a Controversy Paperback - January 15, 2012
https://www.amazon.com/Mohammed-Charlemagne-Revisited-History-Controversy/dp/0578094185/ref=sr_1_1?s=books&ie=UTF8&qid=1468674054&sr=1-1&keywords=Mohammed+%26+Charlemagne+Revisited