"...when you find your soul, you have to go"

And someone (something?) leaving is the trigger for the wildly creative fantasy Summer and Bird by Katherine Catmull.  Summer and Bird are sisters, and they wake up one morning to find that their parents are gone.  Trying to follow a picture-letter left behind by their mother, the girls stumble into Down, a place of magic.  They're soon separated, and as the narrator follows their individual journeys, the reader learns that Mom is actually the queen of birds and can change from woman into bird and back again by using the swan robe.  The girls encounter ravens, snakes, the evil Puppeteer, and so many other fantasy elements that your head will spin.  Imagery! Symbolism!  Metaphors!  The language is rich and the details are intricate, but the story gets so unusual (a World Snake?  a woman who eats live birds?) that I think this book is better suited to older readers.