An interesting book to read (although totaly fiction) is Madeleine L'Engle's "Many Waters" which is about the twin borhters of Meg Murray from "A Wrinkle in Time" (probably her best known novel).
But it deals with the twins going back in time to just before the great flood happened. And they just happen to land in with Noah and his family.
Madame L'Engle obviously uses the Bible (and probably the Apocrypha - some books of the Bible recognized by the Catholic church, but not by all Protestant churches) as a source for her book, as well as her own imagination. But it gives you a really neat feel for what it might have been like back then.
This is the time period just before the Nephilim were supposedly wiped out. And they do feature in the book. It is very neat. Although, like I said, fictional.