Death Cloud (Sherlock Holmes: The Legend Begins, Book #1) by Andrew Lane
It is the summer of 1868, and Sherlock Holmes is fourteen. On break from boarding school, he is staying with eccentric strangers—his uncle and aunt—in their vast house in Hampshire. When two local people die from symptoms that resemble the plague, Holmes begins to investigate what really killed them, helped by his new tutor, an American named Amyus Crowe. So begins Sherlock's true education in detection, as he discovers the dastardly crimes of a brilliantly sinister villain of exquisitely malign intent.