Starters by Lissa Price

In a post-apocalyptic future where a plague has left society divided between vulnerable teens and wealthy elders, a desperate girl rents out her body to an organization that allows seniors to inhabit young hosts. When a malfunction leaves her conscious during a rental, she discovers a sinister plot to control and permanently steal bodies. Caught between a charming boy with secrets, a ruthless mastermind known as the Old Man, and her need to save her little brother, she risks everything to expose the scheme and reclaim her life.

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