The sword thief - Peter Lerangis

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.Think
you've gone everywhere books can take you? Think again. Books are
going to a new place, but you have to follow the clues to get there.
Join Amy and Dan as they continue the hunt for the 39 Clues in Book
Three: The Sword Thief. The high-octane adventures will continue
for a total of ten exhilarating books written by well-known authors
over two years.
The 39 Clues also encompasses hundreds of game cards and an online
world that that allows you to play a part in the story and compete
for 100s of prizes, totalling over +50,000.