Vespers Rising by Rick Riordan, Peter Lerangis, Gordan Korman, and Jude Watson

This book serves as a decent transition from the 39 Clues (where Dan and Amy race to assemble a serum) and the new series where they have to struggle against the Vespers. The story follows a mysterious ring through four generations (Gideon, Madeleine, Grace, and Dan and Amy)--and while you can tell that each section was the responsibility of a different author, the story fits together reasonably well.