The 3rd and final book in the Until the End of the World series.

The books, in order:
Until the End of the World (Book 1)
So Long, Lollipops (Book 1.5)
And After (Book 2)

All the Stars in the Sky:

Cassie Forrest has sworn she’ll never let the world get the best of her again.
She’s chosen to believe everything will be all right.

But on a journey filled with heartbreak and madness and zombies, Cassie and her friends must struggle to stay alive—and it’s hard to believe in a future when survival seems unlikely.