The Wheel of Darkness by Douglas Preston & Lincoln Child
I'm still reading this all out of order. Got this one from the library sale (apparently, it fell in the tub or was left in the rain or something since it was badly water damaged). Even though I missed out on something momentous in the last book, that really didn't impact my ability to follow this one.
It opens with FBI agent Aloysius Pendergast heading to a monastery in Tibet with his ward, Constance Greene. They're there to heal from whatever happened last book and Constance, in spite of there never being a woman taught at this monastery, is taken on as a student. However, Pendergast quickly finds out that there in a catch to this. The monks have had something stolen from them, the Agozyen, something the monks have guarded for centuries.
Pendergast leaves to find this for them and Constance stays behind long enough to suss out why the monks need it back so desperately even though they have never looked in the box that holds the Agozyen. She catches up with Pendergast just in time to board the Britannia, a luxury ocean liner on her maiden voyage. The Agozyen is a force of evil designed to cleanse the world once it become corrupt (and what could be more corrupt than million and billionaires on a cruise ship?)
Trapped on board with the Agozyen and the killer who stole it, Pendergast and Constance have to find the culprit but it's quickly obvious they're too late to stop him from taking the Agozyen from the box as crew and passages' begin to get killed or go mad. Locked aboard a ship, Pendergast exposed to this ancient evil, it's a race to see what wins, good or evil.
All in all, it's a good read but not great. There are others in this series I've liked much better. Part of my problem with this is the plot relies heavily on someone in power acting incredibly stupid. It wouldn't have worked otherwise and I hate that sort of plot device. Also, it spends a lot of time with the various crew and passengers. While they're interesting, Pendergast takes the back seat many times in his own story. It's worth the read but I didn't love it.