The series is about two orphans from Boston, named Amy and Dan Cahill. Their ancestors were some of the worlds most famous people, including Benjamin Franklin, Mozart, Elvis, and George Washington. The series is a big hunt for 39 clues, which if found, can make you become the most powerful person in human history.
There are 11 books in the series, as far as I know. Book 11, "Vespers Rising", comes out on April 5th, 2011. The books take place around the world, from as far south as Australia to as far north as England, and everywhere in between.