The Alchemist's Cat written by Robin Jarvis
This is a prequel to the Dark Portal also written by Robin Jarvis. It is Book One of the Deptford Histories. The Alchemist's Cat is for older readers, about 5th grade and up. It is a dark story of a young boy, Will Godwin, newly orphaned. He travels to London in 1664 with his neighbor, a miller named John Balker. Miller Balker goes to London on a two-fold mission. First to accompany Will and deliver him to his uncle Samuel Godwin, second to look for his daughter who ran away to London many years before. Tragedy, in the form of a murder, occurs. Young Will finds himself assistant to a wicked alchemist known as Elias Theophratus Spittle. This alchemist is truly evil, as he searches first for gold, then for the philosopher's stone, and finally for the elixer of life.
The Alchemist's Cat is the beginning. It explains how the cat known as Jupiter came to be so powerful and full of evil. It is a tale of wickedness, sorcery and treachery. It takes place during the time of the Black Plague in London, and is full of the darkness of that time.
I have not yet read the Deptford Histories which chronicle the battles between Jupiter and the mice of Deptford. If others have, please post your thoughts on these: The Dark Portal, The Crystal Prison, and The Final Reckoning all written by Robin Jarvis.