Into the Gauntlet is the tenth and final book in The 39 Clues novel series. It was written by Margaret Peterson Haddix and released on August 31, 2010. In the beginning of the book, Amy and Dan think about if they ever should have joined the clue hunt, and every misfortune they've had. Their lead is a poem found in their hotel room. However, it is then stolen by Isabel Kabra. It is revealed that William Shakespeare was a powerful Madrigal and the most important clue (the serum recipie) is on the line against their competitors. Mr. McIntyre and Fiske Cahil reveal there's another family out there that makes Isabel Kabra look like Mother Teresa. The globe on the 39 Clues symbol is (Unlike the first nine books) shattered into pieces.
The message on the cards is: Only together can we fight the true enemy.
The secret message in the book is: The Cahills aren't the only ones looking for the Clues. The Vespers are coming.