The Infinite Sea by Rick Yancey (The 5th Wave #2)

How do you rid the earth of seven billion humans? Rid the humans of their humanity.
Cassie Sullivan and her companions lived through the Others' four waves of destruction. Now, with the human race nearly exterminated and the 5th Wave rolling across the landscape, they face a choice: brace for winter and hope for Evan Walker's return, or set out in search of other survivors before the enemy closes in. Because the next attack is more than possible - it's inevitable.
No one can anticipate the depths to which the Others will sink - nor the heights to which humanity will rise . . .