God forms

What is a good book that discusses how to assume a god form?

Assumption of godforms is a part of the Golden Dawn and Thelema systems of ceremonial magick so you should find information on the process in any book detailing those traditions.

It’s also covered in The Magician’s Workbook by Steve Savedow (who I think is a Golden Dawn magician).

Another book that talks about it is The Chaos Protocols by Gordon White (the author uses the technique to assume one of the fictional “Annunaki” talked about by Sitchin).