Frances Hardinge: Twilight Robbery [2011] paperback

The city at night is a dangerous place...Mosca Mye and Eponymous Clent are in trouble again. Escaping disaster by the skin of their teeth, they find refuge in Toll, the strange gateway town where visitors may neither enter nor leave without paying a price. By day, the city is well-mannered and orderly; by night, it's the haunt of rogues and villains. Wherever there's a plot, there's sure to be treachery, and wherever there's treachery, there's sure to be trouble - and where there's trouble, Clent, Mosca and the web-footed apocalypse Saracen can't be far behind. But as past deeds catch up with them and old enemies appear, it looks as if this time there's no way out...