Terrestials (Complete Duology Omnibus of Terra and Underground) by Gretchen Powell Fox:
 
A world split in two. A mysterious stranger. A discovery that will change everything.

After a brutal, sweeping plague decimated the Earth, the world is split in two. Above, the elite live in opulent skycities, while those who remain on the ground—terrestrials—struggle in settlements strewn across the dying planet.

Eighteen-year-old Terra Rhodon ekes out a meager living as a scav, scouring the earth for skycity scraps and doing whatever it takes to keep her younger brother safe. But when a routine scavenging run uncovers a dangerous secret—and an even more dangerous skydweller—Terra finds herself pulled into a conspiracy that could shatter everything she thought she knew about the world, her family, and herself.

TERRESTRIALS brings together both books of Gretchen Powell Fox’s YA dystopian sci-fi duology—Terra and Underground—for the first time. Perfect for fans of The Hunger Games, Divergent, Shatter Me, and The Lunar Chronicles.