The 39 Clues: The Sword Thief by Peter Lerangis

Okay, so this isn't a YA book, but a girl's gotta branch out some times. I love this series. If you like action and adventure mixed with a good healthy dose of sibling rivalry and hijinks, you should definitely check these out.

Here's a short description of this one, which is the third in the series.

ATTENTION! Amy and Dan Cahill have been located once again, this time in the company of the notoriously unreliable Alistair Oh. Could they have been foolish enough to make an alliance?
Spies report that Amy and Dan seem to be tracking the life of one of the most powerful fighters the world has ever known. If this fearsome warrior was a Cahill, his secrets are sure to be well-guarded . . . and the price to uncover them just might be lethal.

I believe there are going to be 12 books in the series and each one is written by a different children's author. Book 1 was by Rick Riordan (The Lightning Thief) and book 2 was written by Gordon Korman (No More Dead Dogs). Part of the fun is seeing how these authors with their varying styles can continue the story of Dan and Amy and their race to find the truth of the Cahill family.

There's also an online game where you can search for clues and solve the big puzzle to win lots of money, but I'm too old to play. Sad isn't it??

Anywho, I love this series and can't wait to see what happens to these two next. It's full of a wonderful casts of characters some of whom you can't't help but love and some who you love to hate. Check these books out, you won't regret it.