Londonia is a magnificently immersive page-turner. Set in 2073, it seems at first to be a dystopia in which the internet and other modern technologies have collapsed. An elite have sealed themselves up in Central London, while everyone else has to get on as best they can, making-do, bartering, and cooperating with their neighbours. Moving between the two societies is Hoxton, a "Finder" of desirable objects, her own past a mystery to be solved, with...