Hi Ian
One of the classic works on this subject (I think long out of print) is Squier & Davis' Ancient Monuments of the Mississippi Valley (1848) which explores many of the great Indian mounds. A more up-to-date survey was the one done by Sue Woodward & Jerry McDonald, called Indian Mounds of the Middle Ohio Valley. (1986) I know there's many more all over the US, but I don't know many other surveys. Paul Devereux describes them and some recent anthropology ideas about them in Earth Memory and Mysterious Ancient America. Check 'em out on eBay.
As for TC's Legends o' t' Sons of God - givvit a miss. Daftest thing he ever wrote - though such ideas were in vogue at the time.
Cheers - Paul