I was born in the same year as Anais Mitchell and had this book from 1985 about Greek Gods and Heroes when I was growing up; I suspect she had this same one. If you're looking for ideas for other musicals based on Greek mythology, this is a good overview and has lovely illustrations. I personally would be interested in seeing the story of Perseus, Medusa and the Gorgons told from Medusa's pov. Or the story of Atalanta, or the Minotaur. Echo, Hera, and Narcissus might be good. Or Pandora. Most of the famous ones have already been told many times over.