The Shadow Thieves by Anne Ursu.
After her cousin Zee arrives from England, thirteen-year-old Charlotte and he must set out to save humankind from denizens of the underworld, Nightmares, Death, Pain, and a really nasty guy named Phil. The world of Greek Mythology comes to life (or death)!
Zel by Donna Jo Napoli.
Based on the fairy tale Rapunzel, the story is told in alternating
chapters from the point of view of Zel, her mother, and the prince, and delves into the psychological motivations of the characters.