The Demon's Parchment

The Tracker pursues mystical documents said to conjure demons while hunting for a monstrous killer in this medieval mystery with "an appealing noirish air" ( Booklist).
Since losing his knighthood, Crispin Guest has reinvented himself as an investigator for hire known as the Tracker. But his reputation will once again be put to the test with his latest case. Jacob of Provençal, a Jewish physician at the king's court, is missing a set of documents that he claims contain the power to bring forth a dangerous demon.
Meanwhile, it seems a monster has already been unleashed on the city. Vulnerable street children are being abducted and murdered, their mutilated bodies the only clues left behind. With the help of his orphaned servant, Jack, it is up to Crispin to unravel the grim tangle of mystery and murder.
This third installment of the Crispin Guest Medieval mysteries was nominated for a Romantic Times Award and was a finalist for the Macavity Award.