The Blood of Olympus by Rick Riordan (The Heroes of Olympus #5)
They are on a tight deadline. They have also been having nightmares of battle at the camp half-blood. They all want to go back there and help their friends, but their quest is more important. So Reina, Nico and the coach shadow travel back west, while the heroes go east.
It's the final book in the series and the stakes have never been higher.
It's a good conclusion to the story, but I was honestly a bit disappointed with the ending. After all that build-up, it seemed a bit underwhelming. The one thing that I consistently didn't like in all of the books was the narration. I didn't like the POV switches, they didn't work as well as the first-person narrative in the first series.