Some of the stuff in there comes from older pagan documents, even Gnostic materials, and parts appear to have the Torah as their source, albeit a Torah or two not exactly consistent with the standard version. (There were Jewish tribes in the Arabian desert in those days. They all had a Torah or two. They buried old worn out Torah's in the dry desert soil where they'd last centuries.
You must also remember the purpose of the Kaabah in Mecca. People throughout the region brought copies of their "gods" to the Kaabah and placed them in a circle around a moderate sized meteorite (Arabia has lots of meteorites on the surface in the Rub al Khalid, the great desert to the East.) They also brought copies of their sacred texts and buried them in and around the area now occupied by Mecca.
It is, in fact, the Arabian tradition that Hagar was sent to the Wilderness at the spring of Zim Zim which is, in fact, the source of water for the oasis at Mecca.
There are caves in the hills where texts could be buried and kept intact for thousands of years.
A couple of Turkish Caliphs had an interest in archaeology and pretty much dug up everything of interest in and around Mecca and Medina so it's unlikely such things still remain. The early Moslems also burned the wooden idols at the Kaabah.
That there was a religious reformer in the time of Mohammad is beyond doubt. That his name was actually Mohammad is questionable since that name doesn't pop up until the Moslems had conquered Damascus (where that missionary's manual came from) many years after Mohammad's death.
The best opinion in the West is that there was a religious reformer, that ancient texts played a part in what he was doing, and there was Damascene Christian influence in the early days of the Moslem movement.
For the first 300 years after Mohammad fled to Medina Moslems appear to have attended Christian churches.
There is much less known about the founding of this religion than is popularly believed. Questioning the official story is considered heresy and worthy of death.
Thanks for the info! As always, your posts are very interesting!