The Drowned Cities by Paolo Bacigalupi

Bacigalupi, author of Shipbreaker, continues his exploration of a dystopian future in which major American cities have been flooded and turned into jungles. Gangs of armed children roam the ruins and surrounding countryside looking for enemies and vulnerable civilians to impress as slaves. Two children have managed to survive the violence, although Mahlia has lost a hand and Mouse is soon captured by a rebel band, with whom he must actively participate in killing and torture or be executed himself. Despite her desire to escape the violence, Mahlia chooses to return to the Drowned Cities to find and rescue her friend. She has one thing in her favor, she doesn't travel alone. Tool, a combination of man, dog, tiger and jackal, has escaped from his masters who first created and then used him as a killing machine. Tool owes Mahlia a debt--she stole medicines to heal his wounds and save his life. Still, he can't really figure out why he doesn't either abandon her or eat her. Tool and Mahlia face constant danger in their quest to find and save Mouse. Meanwhile, Mouse's captors are turning him into Ghost--an armed and ruthless soldier like themselves.The story unfolds with visceral violence and unending suspense.