When a mysterious letter written in code arrives at King's house for his human, Kayla, he follows the trail of the person who left it.

When Amy and Dan Cahill trace the footsteps of a formidable military leader from the Cahill bloodline, they uncover a devastating secret about the family that changes everything.

An escalating race to find the 39 Clues finds Amy and Dan pursuing a Clue guarded by thousands of the world's best-trained soldiers, an effort that separates them in explosively dangerous ways.

When the Vespers continue their abductions and cross a line by kidnapping Atticus, an eleven-year-old non-Cahill civilian and Dan's only friend, Dan and Amy confront their biggest challenge ever in order to keep Atticus alive.

A guide to the characters and situations in "The 39 Clues" outlines the Cahills' secrets, including hidden facts, strategies, agents, lost founders, secret bases, and scandals, as well as information about all branches of the family.

Returned by a surprising twist to their first mission on U.S. soil since the beginning of the Clue hunt, Amy and Dan discover evidence of a mole in their organization who proves to be one of their closest confidantes and who reveals, even more disturbingly, the true agenda of the Vespers.

As Cruz and his friends continue to search for the third piece of the cipher, a mysterious message arrives suggesting that someone close to him is working for Nebula, leaving Cruz unsure of who he can trust.

During World War II, eleven-year-old Felicity is sent from London to Bottlebay, Maine, to live with her grandmother, aunt, uncle, and a reclusive boy who helps her decode mysterious letters that contain the truth about her missing parents.

"Amid assignments that take the Explorer Academy recruits from the iceberg-filled waters of Antarctica to the bone-dry deserts of Argentina, Cruz Coronado is scrambling to complete the last piece of the cipher. With Nebula agents and the elusive explorer spy still out there, his opportunity to recover his mother's world-changing formula is slipping away. But as Cruz has learned from his time aboard Orion, true explorers must never give up. Even after...

When Iranian-American Kamran Smith learns that his big brother, Darius, has been labeled a terrorist, he sets out to piece together the codes and clues that will save his brother's life and his country from a deadly terrorist attack.