Reads like Luke W. was the ghostwriter. (Which means it's really good!) Starts with a bang! 
 I read this one pretty quickly, as it is definitely a page-turner. My expectations may have been excessively high, due to the fact that I like Rick Riordan and I've heard so much buzz about Scholastic's hope to keep themselves big in the post-Harry Potter market with this new "multi-platform" series (books, trading cards, internet game, and cash prizes). I'm giving this four stars and not five for the facts that 1) all these efforts seem a bit forced, 2) they require purchase and/or computer access, 3) Rick Riordan still has a knack for capturing the middle-school mindset and dialogue, but this book's frequent mention of iPods and celebrities like Kanye West are going to date it, and 5) dang the cliffhanger ending, because in spite of it all, I want to know what happens!!! 
 What I really like about this book, as with the Percy Jackson books, is the seamless integration of a fair amount of historical fact and nonfiction. The main characters, Amy and Dan, spend a lot of time in libraries (much to Dan's dismay). This book does not have an alternate world like Hogwarts, and while there are sometimes *too* many ties to the world of 2008 (again with the iPods), Riordan takes full advantage of reality to draw upon history and science. 
 I'd call this a little like "The Da Vinci Code" for kids. Incidentally, I did join online (www.the39clues.com) and found that I am a member of the Lucien branch (I have a bad feeling this is the equivalent of being sorted into the Slytherin House, but we shall see). I also got a confirmation that said "check your email and if you don't see one, check your junk folder. You may have been sabotaged." Ha!