I guess that Spencer is using research done by Sven Kalisch http://en.wikipedia.org/wiki/Sven_Kalisch that formerly was a Muslim scholar, and since that is impossible be skipped the muslim part:
Doubt about Muhammad’s Existence Poses Threat to Islamic Religious Education 18 sept 2008
http://www.freerepublic.com/focus/f-news/2088564/posts
“I guess that Spencer is using research done by Sven Kalisch “
Among others. I wonder if there’s really anything new in his book, or he just needed a bigger payday?