The Mansion in the Mist — John Bellairs

Anthony Monday takes a vacation to another world in this mind-blowing mystery from the author of The Lamp from the Warlock's Tomb.
 
Summer is here and Anthony Monday has been asked to join his friend Miss Eells and her brother Emerson on a vacation to northern Canada, where Emerson owns a no-frills cottage on an island. School may be out, but there's still a riddle to be solved: A few years ago, three tourists visiting the cottage disappeared without a trace . . .
 
The cottage has no electricity, but it's humming with strange sounds and illusions. Before long, Anthony finds a disappearing chest that turns out to be a portal to another world -- a realm populated by evil human-like creatures who want to drag Earth and its inhabitants into their dimension. As Anthony, Miss Eells, and Emerson try to come up with a plan to save the world, they are faced with their own intruder: a visitor from the other side with vengeance on his mind . . .
 
“The atmosphere throughout this adventuresome chiller is appropriately scary and the villains are certainly evil personified.” -- School Library Journal
 
“Bellairs's characters have a captivating charm that adds to this spellbinding adventure.” -- VOYA
 
“Throwing in plenty of conventional ingredients (ghosts, illusions, cryptic clues, secret passages, magic amulets, a witches' sabbath, cliffhangers, last-instant rescues, etc.), Bellairs dishes up a broth spiced with action [and] suspense.” -- Kirkus Reviews
 
“While the notion of passage into another world is not new, the late Bellairs provides unique twists.” -- Publishers Weekly