|The Black Book of Buried Secrets|
|Publication date||October 26, 2010|
The Black Book of Buried Secrets is an extra book that is not a part of any series involving the Clues. It was released after the Clue Hunt series was finished, and is closely associated with that series. The book offers details on the branches and their leaders, along with old branch secrets that are not mentioned in the Clue Hunt books. Rick Riordan wrote the introduction of the book, and Mallory Kass wrote the rest.
"The book you see here is the ultimate source of all Cahill knowledge. It contains every buried secret, Cahill weapon or gadget, all the strongholds, and each agent and founder. In it, you will discover the true story of Madeleine Cahill as well as what happened after Isabel Kabra was charged with murder. Nothing has been held back.
Seven Cahill agents came together to bring you this book, betraying their branches and their most deeply held beliefs. They did it because there is an enemy approaching far more powerful than the Cahills. Keep this black book close at all times, and be ready."