Vespers Rising is the eleventh and final book of the original The 39 Clues series, a collaborative novel written by four of the series' authors: Rick Riordan, Peter Lerangis, Gordon Korman, and Jude Watson. It bridges the original ten-book clue hunt and the sequel series, Cahills vs. Vespers, revealing the hidden history of the Cahill family and the shadowy enemies who have stalked them for centuries: the Vespers. The book is structured as four short stories spanning five hundred years, each by a different author. Rick Riordan opens the book with the origin story set in 1507, recounting how Gideon Cahill invented the master serum during the Black Death, and how his friend turned betrayer, Damien Vesper, brought down Gideon's laboratory to seize it, scattering the serum's ingredients among Gideon's children. The second story follows Gideon's youngest daughter Madeleine, founder of the Madrigal branch, in 1814. The third, set during World War II, shows the children's grandmother Grace Cahill as a fourteen-year-old in action. The final present-day section returns to Amy and Dan Cahill in Boston, living with great-uncle Fiske and au pair Nellie Gomez, as the Vesper threat resurfaces and Grace leaves them one last, dangerous legacy. The result functions both as a satisfying epilogue to the clue hunt and a prequel that sets up the next chapter of the saga.