Every time I read one of the 39 Clues books, I am amazed at all the cultural and historical details the authors weave into the story. In Amy and Dan Cahill travel to: Italy, Switzerland, Germany, and the Czech Republic. Their hunt involves facts about Marco Polo, medieval world maps, the theft of art by Nazis during WWII, Neuschwanstein Castle, Johannes Kepler, and Tycho Brahe.
The series reminds me of several famous books/movies for adults. There are the gadgets, computers, exotic locales, and the villains with their sinister organization that you might expect in a James Bond or Lara Croft adventure. Then you have the codes and historical references that are similar to what Robert Langdon deals with in Dan Brown's books, or the clues Nicholas Cage pieces together in the "National Treasure" films.
When you put it all together in one place you get a series with action, adventure, and mystery, plus the added bonus of the online game. It's a win-win situation. Check out the website to play the online game and find out more information about the books, their authors, and new series Unstoppable that is coming in October 2013.