Magic in the Pool of Making by Beth Webb
Years of pollution and misuse have put the River Planet in danger. Manny, a mysterious and starving Sand-boy is the only person who knows how to put it right. Now, through a simple act of kindness, Johin (always accompanied by her pet mouse Nuffle) is entwined in a dangerous quest set by this charismatic stanger. Together, they race against time and the evil Brilliance to save the planet.