While reading through some weekend posts from blogs that I follow, I came upon one that piqued my interest:
Like Allan, I too am quite "interested in the origins of Islam". Back on Dec. 2, 2011, I published a thread that listed a number of early works (632-900 A.D.) contained in the book: The Early Christian-Muslim Dialogue: A Collection of Documents from the First Three Islamic Centuries (632 - 900 A.D.): Translations with Commentary. [LINK TO THREAD.]
The document quoted by Allan, Doctrina Jacobi, was not included in the book. However, even though Allan did not provide the source of his quote, I immediately recognized that I had seen it before, and was pretty confident that it was in Robert G. Hoyland's massive, Seeing Islam As Others Saw It. I pulled the book off of the self, and sure enough, found the quote—word for word—on page 58.
Hoyland's book is a must read for those who are interested in the early history of Islam. I was going to provide a Google Books link to the book, but to my surprise, found out that a PDF version is available for free via the following link:
Grace and peace,