The Sisters Grimm series by Michael Buckley:
The Mysterious Benedict Society series by Trenton Lee Stewart:
Puzzles! Clues! Riddles! Wordplay! Responding to an advertisement recruiting "gifted children looking for special opportunities" Reynie, Kate, Sticky, and Constance are orphans who end up in the Mysterious Benedict Society (trained by the mysterious Mr. Benedict himself) and are flung headlong into a series of kinds of suspenseful adventures.
The Secret Series by Pseudonymous Bosch:
Each of these books is based one of the five senses and secrets abound! The last one You Have to Stop This - based on touch- ends with a cliffhanger....even though there has been no confirmation of a sixth book! Bosch is secretive like that. In fact, everything is a secret in these books (even the protagonists have fake names) so make sure a kid who loves being befuddled gets 'em. And check out Bosch's website for even more confusion!
