The Traitor’s Blade by Kevin Sands, the 5th book in the Blackthorn Key series. This terrific middle grade series for older elementary/teen readers (and people like me!) features an apothecary’s apprentice in London in the mid-1600s. Every book finds Christopher and his friends using chemistry and ciphers to help solve a crime or mystery. I love these because they mix historical fiction with adventure with the bonus of learning about decryption techniques and science of the times.