Ice breaker by Lian Tanner

The Hidden series, bk 1. Allen and Unwin, 2013. ISBN 9781743314340.
(Age: 11+) Highly recommended. Adventure, Fantasy. The first in The Hidden series, introduces Petrel, a young girl who survives by her wits in a strange world aboard a huge ship where three tribes have developed, markedly separate from each other, the Cooks, Engineers and Officers. She avoids each level of the ship, using the passages behind the metal of the ship to crawl from one level to another, with her two rat friends Mister Smoke and Missus Slink.
Known as Nothing Girl to all tribes they tease, abuse and chase Petrel when she is sighted, as she is the daughter of a forbidden relationship, between two people from two levels, for which crime they were thrown overboard. This is an even nastier world like the one created by Tanner for her last unputdownable series, The Keepers.
Onto this ship comes a small boy who Petrel calls Fin. Found on the ice he needs to be nursed back to health, but a stranger on board causes increasing tension between the ship's tribes, and when the leader is killed, war breaks out.
Petrel scurries Fin to a safe haven but she is found by Squid, the daughter of Seal, another leader, and together they try to hide Fin after his talk while delirious shocks them all. He is the harbinger for a ship following, intent on destroying the ship and all of its inhabitants. So time is of the essence.
Petrel and the rats seek out the 'sleeping captain', the story of a man who will save the ship, and so through the secret tunnels and cavities of the ship they travel, with war and now fire raging on every level.
This is an exciting adventure story, set in a blizzardly cold place, where to go on deck means having to wear very protective outdoor gear, to be thrown overboard means a quick death from the cold. Petrel's life is always precarious, but when she defends the found boy, she is targeted by everyone. With a steampunk background, the mechanics of living in such a world is well drawn, making a wonderful background to the story which will capture the imagination of any reader, but particularly those absorbed by fantastical worlds.
Fran Knight