Vespers Rising: 10/07/13
Vespers Rising by Rick Riordan, Peter Lerangis, Gordon Korman, and Jude Watson is the bridge between the original 39 Clues series and the Vespers vs. Cahills.
Rather than being just an adventure with Amy and Dan, it's four short stories. It takes Cahill family through important eras: the loss of the family home, the rise of the Madrigals, Grace Cahill's first adventure, and finally a present day adventure with Dan and Amy.
My favorite two adventures were the ones with Grace Cahill and her grandchildren. While Grace's adventure might seem far fetched for a young teen during WWII, it was still more of a romp than the either of earliest stories. Grace's adventure also highlights more so than the previous two the ways in which the Vespers have operatives almost everywhere.
For anyone interested in jumping right back into Dan and Amy's adventures, Vespers Rising can be skipped.