Baldwin Hills – Simon, King of the Witches (New English Library, 1972) [film tie-in]
“Simon was in bed when he first discovered the amazing power he possessed. The potency of his magic led to his rapid rise in the world. His new-found necromancy enables him to bewitch any woman he likes for as long as he likes. And he owes it all to his bewitching instrument!”
Thanks to Steve Goodwin for the blurb and cover scan!