The sinister booksellers of Bath / Garth Nix.

There's often trouble of a mythical sort in Bath. The booksellers who police the Old World keep watch there, particularly on the entity who inhabits the ancient hot spring. Yet this time it's not from Sulis Minerva that trouble starts. It comes from the discovery of a sorcerous map, leading left-handed bookseller Merlin into great danger. A desperate rescue is attempted by his sister the right-handed bookseller Vivien and their friend, art student Susan Arkshaw, who's still struggling to deal with her own recently discovered magical heritage. The map takes them to a place separated from this world. But this is only the beginning, as the booksellers investigate centuries of disappearances and deaths and try to unravel secrets of the murderous Lady of Stone, a serial killer of awesome powers. If they don't stop her, she will soon kill again. And this time, her target is not an ordinary mortal.