Night's Daughter

Since time immemorial, the Kingdoms of Sun and of Night have been at war. Pamina, daughter of the Starqueen, supreme symbol of the Night, and of Sarastro, King of the Royal House of the Sun, must now choose which of her parents' ways she will follow. Together with her love, Prince Tamino, she must face the Ordeals at the Court of Wisdom, of Earth and Air, of Fire and Water. Only if she and Tamino pass all these trials can they have a life together. 

If this sounds like the plot of the opera Die Zauberflöte (The Magic Flute), that's because it was one of MZB's favorite operas - and the inspiration for this book.