Uglies by Scott Westerfeld

In a future society where everyone undergoes surgery at sixteen to become a "Pretty," Tally Youngblood is eagerly awaiting her transformation. However, her world is turned upside down when she befriends Shay, a rebellious girl who questions the system and runs away to join a group of renegades living in the wilderness. Tally is coerced by the authorities to find Shay and the rebels, but as she delves deeper into their world, she begins to question the true cost of beauty and conformity, ultimately facing a life-altering decision that challenges her beliefs and the society she once longed to be a part of.