The Sinister Booksellers of Bath by Garth Nix

There is often trouble of a mythical sort in Bath. The booksellers who police the Old World keep a careful watch there, particularly on the entity that inhabits the ancient hot spring.

This time trouble comes from the discovery of a sorcerous map, leading left-handed bookseller Merlin into great danger, requiring a desperate rescue attempt from his sister, the right-handed bookseller Vivien, and art student Susan Arkshaw, who is still struggling to deal with her own recently discovered magical heritage.

The map takes the trio to a place separated from this world, maintained by deadly sorcery and guarded by monstrous living statues. But this is only the beginning. To unravel the secrets of a murderous Ancient Sovereign, the booksellers must investigate centuries of disappearances and deaths. If they do not stop her, she will soon kill again. And this time, her target is not an ordinary mortal.