Except the parts where it's not the Biblical stories, and that's most of the book.

Things that are not from the Bible can be found in:

1. The Talmud (he who has saved a life has saved all mankind)
2. Apocryphal Jewish sources (Some tales about Solomon, possibly Abraham and the fire)
3. Apocryphal Christian sources (Jesus turning a clay bird into a live on, )
4. Random Christian legends like the Sleepers of Ephesus (The Sleepers)
5. Byzantine legends like the Syriac Alexander Legend (Dhul-Qarnayn)

I wouldn't say it's most of the book because there's a ton of tedious divine hectoring, rulings made for Mohammed's time, insisting that Mohammed is not possessed and not particularly distinct monotheist ranting and so on.

But...it's a lot. The author...allowed themselves to be inspired by whatever they saw in the world.