Day of Doom by David Baldacci

This book concludes the Cahills v. Vespers chapter of the 39 Clues. It wraps up a lot of loose ends (some in frustrating ways). Dan, Amy, Atticus, and Jake race to stop a doomsday machine--and there's less of the charming historical detail and discovery because there's so much action to fit in and so many plot lines to tie up.