Maybe a Fox - Kathi Appelt - Bog

When Sylvie runs to the river she and her sister Jules are not supposed to go anywhere near to throw a wish rock just before the school bus comes on a snowy morning, she runs so fast that no one sees what happens ... and no one ever sees her again. Jules is devastated, but she refuses to believe what all the others believe, that--like their mother--her sister is gone forever. At the very same time, in the shadow world, a shadow fox is born--half of the spirit world, half of the animal world--and she senses something is very wrong.