The Cinderella Theorem — Kristee Ravan

Fairy tales are naturally non-mathematical. That is a fact, and fifteen-year-old Lily Sparrow loves factual, mathematical logic. So when her mother confesses that Lily's deceased father is (a) not dead, (b) coming to dinner, and (c) the ruler of a fairy tale kingdom accessible through the upstairs bathtub, Lily clings to her math to help her make sense of this new double life (1 life in the real world + 1 secret life in the fairy tale world = a double life). Even though it's not mathematical, Lily finds herself being pulled into a mystery involving an unhappy Cinderella, a greasy sycophant called Levi, and a slew of vanishing fairy tale characters. Racing against the clock, with a sound mathematical plan, can Lily save her fairy tale friends before they vanish forever?