Original Title: Herr der Diebe
I think this novel might be aimed at children a bit younger still than the Inkworld-trilogy. But, if you can put aside the fact that it is completely unbelievable, it's a very enjoyable read.
It's about a bunch of kids in Venice, who have their own underground society. Their leader is a masked master thief.
It's a very original story, and the writing is nice as well. It reads quickly but the story perhaps is a bit childish (and that's something you either like or not).
Note: I read a Dutch translation of this book