Philip Reeve 
Utterly Dark and the Face of the Deep 

It was always at sundown they were seen. In that twilight hour, when the walls between the worlds grew thin, strange things might slip through the cracks. Sometimes then, so the stories went, enchanted islands would appear in the empty ocean to the west of Wildsea.When Utterly Dark was a baby, she was washed up on the shores of the Autumn Isles and taken in by the Watcher of Wildsea. But everything changes when her guardian suddenly drowns. Now who will keep the Watch, and make sure Wildsea stays safe from the strange forces teeming in the deep ocean around them?A magical new story from the bestselling and prize-winning author of Mortal Engines.