Barbarossa has a red beard in the books but not in the movie
Bo is five years old in the book but in the movie he says himself that he is "6 and a quarter."
Mosca's father wasn't mentioned in the book, however in the story, he has a father who was lost at sea.
Victor meets Ida towards the end of the book, but in the movie they knew each other from the start.
The magical elements are introduced earlier in the movie.
In the novel Prosper and Bo are brought to the Thief Lord by Hornet but in the movie, Scipio discovers them when they were being chased by a pharmacist when Prosper was trying to shoplift medicines for Bo.
Near the end of the book Barbarossa tries the roundabout and brakes the wing off, but in the movie when Barbarossa tries it the whole roundabout ends up in flames.