The bone snatcher / Charlotte Salter.

Enhanced descriptions from Syndetics:

Murder, madness, and sea monsters combine in this thrilling and atmospheric middle grade debut perfect for fans of Neil Gaiman, Lemony Snicket, and Tim Burton.

Sophie Seacove is a storyteller. She tells stories of what the world would be like if madness hadn't taken over. If her parents hadn't sold her off as a servant to pay for their stupid vacation. If she wasn't now trapped in a decaying mansion filled with creepy people and surrounded by ravenous sea monsters.

The mansion has plenty of stories, too- About fantastical machines, and the tragic inventor who created them. About his highly suspicious death. And about the Monster Box, a mysterious object hidden in the house that just might hold the key to escaping this horrible place-and to reuniting Sophie with her family.

But not everyone wants Sophie to have the Monster Box, and as she gets closer to finding it, she finds herself unspooling years-old secrets-and dodging dangerous attacks. Sophie needs to use her brains, her brawn, and her unbreakable nature if she wants to make it off this wretched island...and live to tell this story.