Mark of the Plague (Blackthorn Key series)

In 1665 London, a plague outbreak devastates the city. Thirteen-year-old Christopher is an apprentice apothecary at Blackthorn’s shop, which is selected to prepare a remedy supplied by an unknown apothecary. Although the cure appears to work, Christopher becomes caught in a conspiracy involving false cures. With his friend Tom, he follows puzzles, riddles, and secrets while an assassin threatens the apothecary’s life. Their investigation risks their lives as they try to uncover the heart of the conspiracy.