Agatha Christie’s spirit must be loving this poisonous new historical entertainment. Eleanor Herman has already enjoyed the success of Sex with Kings and Sex with the Queen, thoroughly researched, gossipy revelations of promiscuity among monarchs and their noble retainers during the Renaissance. She is an American author and broadcaster, born in Baltimore, now living in Virginia, but, at 58, she still concentrates her professional attention on the historic immorality and disastrous vulnerability of western European royalty.