The Palace of Laughter by Jon Berkeley

Miles Wednesday, orphaned, unwashed and living in a barrel, has never been to a circus before—but then the Circus Oscuro is no ordinary circus. There's a strange beast called The Null and an array of sinister-looking clowns, and when an unusual little girl with wings falls from a tower during the performance, Miles's life is changed forever. As Miles and Little embark on an extraordinary journey to rescue two friends who have been captured at the Palace of Laughter, they discover nothing less than the power of friendship and the gift of family.