According to the blurb published with this article, a new historical novel about the future King Edward VIII is "cashing in on Wills and Kate hysteria."
However, The Golden Prince by Rebecca Dean was published in the UK in November 2010, several days before Prince William's engagement to Kate Middleton was announced. Unless the writer and publisher have much better crystal balls (or time machines) than most royal watchers, it is impossible that this novel (which is getting good reviews on Amazon) was created to exploit the current British royal engagement, oops, I mean "hysteria."
As for The Celestial Voice of Diana by Rita Eide… well, it is out of print, but Amazon has used copies if you really want to read it.