The City in the Middle of the Night by Charlie Jane Anders

January is a dying planet–divided between a permanently frozen darkness on one side, and blazing endless sunshine on the other. Humanity clings to life, spread across two archaic cities built in the sliver of habitable dusk.

But life inside the cities is just as dangerous as the uninhabitable wastelands outside.

Sophie, a student and reluctant revolutionary, is supposed to be dead, after being exiled into the night. Saved only by forming an unusual bond with the enigmatic beasts who roam the ice, Sophie vows to stay hidden from the world, hoping she can heal.

But fate has other plans–and Sophie's ensuing odyssey and the ragtag family she finds will change the entire world.