Click here for a review of The 5th Wave I wrote in 2013. I really enjoyed the first book, and the second took me a little while to get into, but I liked it just as much as the first one.
The sequel, like its predecessor, is told from the point of view of children and teenagers who survived the plague that killed most of the people who lived on Earth. The plague was caused by an alien species called the Others who want to inhabit the Earth. The story begins from the point of view of Ringer, a female teenage soldier. After the events that culminated in the destruction of Camp Haven, our main characters have holed up in a decrepit hotel, and need to move before they are discovered by the Others and killed.
One of the driving themes of this particular story is the desperate quest to maintain a part of your humanity in the face of the complete destruction of the world. The only reason to fight so hard to continue on is to preserve the part of us that makes humanity worth saving, although it would be easier to continue the fight and to survive without it. It's an interesting dichotomy that is presented over and over again in these stories. The kids have had to become soldiers, and their teachers have attempted to rid them of everything that makes them human, but at the same time they are still just kids, which is obvious in their interactions and when they let their guards down. The story does have echoes of Ender's Game in that respect. But there is a huge reveal in this book that renders that comparison no longer valid. Yancey could have just used this book to further the story and reunite Cassie and Evan, the couple from the previous novel. But he doesn't, he furthers the story so that all of the main characters are closer to the truth, which is something they had never imagined.
The Infinite Sea is available on sale in hardcover for $16.99 as is its predecessor The 5th Wave. They're good books certainly worth your read in the post-apocalyptic tradition of the Hunger Games, Divergent and the aforementioned Ender's Game.