The Brass Key (The Book of Lowmoor series)

In a world shaped by the scattered remains of a giant civilization live communities of six-inch tall human beings. Blackmailed by the sinister Spetch twins, Jewel and Thorn must journey across land and water to retrieve the mysterious brass key. Until they find it and return it to the Spetches, Thorn's sister Haw is at the twins' cruel mercy. Jewel's strength as a Magian grows when she comes across a crystal similar to the one stolen and hidden away by Thorn. It has a strange, dark power—but is there danger in its strength? Encountering unexpected, malevolent forces along the way, but also making new friends, Jewel and Thorn discover a new and even more dangerous opponent lurking in the shadows behind those they already know.