From the ingenious ciphers of Italian princes to the spy books of the Civil War to the advanced techniques of the CIA, codes and code breaking have played important roles throughout history.

A Clue found in Book 3 sends Amy and Dan jetting off to find out just what's behind the fierce rivalry between the Tomas and Ekaterina branches of the Cahill family. Was a Clue stolen from the Tomas branch? Where is it now? And most important, can Amy and Dan get their hands on it before their rivals do?

Cahills are being kidnapped by a shadowy group known only as the Vespers. Now Amy and Dan have only days to fulfill a bizarre ransom request or their captured friends will start dying. Amy and Dan don't know what the Vespers want or how to stop them. Only one thing is clear. The Vespers are playing to win, and if they get their hands on the Clues ... the world will be their next hostage.

"Cruz and his fellow recruits tackle challenging missions to dispense life-saving medication to gorillas, thwart pangolin poachers, and capture images of the last known cheetahs in Namibia--all the while discovering that protecting the world's threatened species is a dangerous business. Things turn deadly for Lani, Sailor, Emmett, and Cruz as they search for the fourth piece of Cruz's mom's cipher. Cruz knows it's not going to be easy, but will an...

"Twelve-year-old Fish--nicknamed for his skill as a swimmer--never intended to become a pirate. For starters, he's just a kid from a poor farming family, and second, he absolutely hates to fight. But after his parents send him to work as a courier, one of his deliveries is stolen, and as Fish hurries to get it back, he's captured by a ship of scalawags. And the mysterious package? It's the key to locating the Chain of Chuacar, a legendary treasure....

A romantic thriller about an Oxford professor who is hired by a mysterious Arab oil magnate to decipher a secret message and befriends his exotic companion.

Amy and Dan Cahill have been located once again, this time in the company of the notoriously unreliable Alistair. 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.

Dr. One-Zero won't stop until the whole town--no, the whole world--embraces the "true happiness" found in his poisonous potion, Green Pop. And now that he has the Turtle of Light, he's virtually unstoppable. There's one weapon that can defeat him: another Turtle of Light. Unfortunately, they can only be found in another dimension! To open a portal to this new world, Hopper, Eni, and Josh's coding skills will be put to the test.