The sixth book in the series. Spoilers ahead for earlier ones.
Especially since it opens with Owen's father revealing the last information to his sons, and collapsing on the floor, and then goes on with the news Owen had gotten, of the trapped survivors in the other world.
The whole process, starting with drawing up a plan and selling it to the others at Monster Hunters International. And outward.
Among other things, this involves a hunt, a magical ring, a talk with a ghost who had been the Chosen, a cyclops, a convoy, using an icebreaker in the summer, bribes, a revelation to a bunch of monster hunters, and a lot more.