The end to Cahills vs. Vespers is full of twists, turns, and changing allegiances to the final moment. Tension comes from all the unknowns - will the hostages be rescued; will they stop the Doomsday device in time; will Dan actually take the serum? Those are just a few of the answers we are searching for as we read. There are shootouts, fights, chases, airplanes, trains, double-crosses, deaths (I'm not saying whose), escapes, and a little romance, too. I enjoy the way bits of history are woven into each book. This time there is a link between the device invented by Archimedes and the famous explorers Lewis and Clark.
What makes this series fun is that there is something for everyone to enjoy: history, science, action, intrigue, romance (that whole Amy/Evan/Jake love triangle), computer sleuthing, and more. I'm interested in what the Cahills will face in the next series. We have all the books from The 39 Clues in the library.