The Stone, the Cipher, and the Shadows (John Bellairs's Johnny Dixon in a Mystery)

A flu epidemic ushers in a plague of dark magic in this spooktastic mystery featuring teenage sleuth Johnny Dixon from The Wrath of the Grinning Ghost.
 
Though forty miles away, Duston Heights is not safe from the flu that’s raging through Boston. When Johnny Dixon’s grandmother falls ill, he’s sent to live with his neighbor to avoid infection. So many locals are getting sick that school is canceled for a week, and the reclusive Dr. Abram Ashburn comes out of retirement to make house calls.
 
After seeing a scary vision of his bedridden grandmother outside of a window, Johnny starts to feel on edge. Then he and his best friend find what looks to be a weird map of a cemetery in Dr. Ashburn’s house. One specific grave is marked with an “X,” the burial place of a woman who practiced witchcraft in the seventeenth century.
 
The townspeople recover from the flu, but they can’t escape the terrifying illusions and shadow people that now haunt them, unless Johnny and his friends find the key to unlock the secrets of the graveyard before a dreadful prophecy comes to pass . . .
 
Praise for The Wrath of the Grinning Ghost
 
“Fans of the series will enjoy this new supernatural adventure, which reads so much like Bellairs’s books that they won’t believe he didn’t write it.” —School Library Journal
 
“Strickland’s story is eerie, suspenseful, and true to the personalities and writing style of Bellairs, who began the Johnny Dixon series . . . This is good reading for adventure enthusiasts as well as for series fans.” —Booklist