I Was a Rat by Philip Pullman

From the back of the book:
"I Was a Rat!" So insists a scruffy boy named Roger. Maybe it's true. But what is he now? A terrifying monster running wild in the sewers? "The Daily Scourge is sure of it. A victim of "Rodent Delusion"? The hospital nurse says yes. A lucrative fairground freak? He is to Mr. Tapscrew. A champion wriggler and a budding thief? That's what Billy thinks. Or just an ordinary small boy, though a little ratty in his habits? Only three people believe this version of the story. And it may take a royal intervention--and a bit of magic--to convince the rest of the world.
Set against the backdrop of a Royal Wedding--and a playful parody of the press, "I Was a Rat! is a magical weaving of humor, fairy tale, and adventure.