Ruby in the Smoke by Philip Pullman

Soon after Sally Lockharts father drowns at sea, she receives a strange anonymous letter. The dire warning it contains makes a man die of fear at her feet. Determind to discover the truth about her fathers death, Sally is plunged into a terrifying mystery in the dark heart of Victorian London, at the centre of which lies a deadly blood-soaked jewel. Paperback , 209 pages Published 2006 by Scholastic (first published 1985)