A King's Ransom by Jude Watson

This book continues the adventures of Amy and Dan, but with a twist. While Vesper 1 still wants them to continue stealing things that lead to a series of medieval maps, Vesper 6 and her twin brother have decided to beat Amy and Dan to the hunt in order to improve their status in the Vesper organization. The book also reveals that the Cahills' father was a Vesper. The story is fun if you don't think too hard about the premise. I also like that the story explores to a certain extent questions of right and wrong (Amy and Dan are doing some terrible things to save their family--but the family code is that they can only do it themselves).