Philip Reeve roams across realms of his own making with effortless brio. If I say that Thunder City (Scholastic, £8.99) is based on the premise that the world’s great cities have detached themselves from their terrestrial foundations and are floating across the sky like so many urban space ships, swallowing up smaller fry, you’d probably think this is asking a bit much of the reader, but somehow you take it all on board.