Let´s talk about Dune...

I have only managed to read the first three books so far and then for one reason or another, I never got around to the others.
Without asking for too much detail, does God Emperor of Dune elaborate further on Leto and his "changes"?
Granted, I am not overly familiar with details of the Middle East(life, religion, etc), but it sure seems like the Fremen are descendants of that culture. That may well be old news to some, but I have always wondered if that were the case with the Fremen.