The Open Door - Margaret Oliphant, Penguin

A classic ghost story illustrated by acclaimed cartoonist, Seth. Retired officer Colonel Mortimer takes a lease on the mansion of Brentwood, the grounds of which share the ruins of an older house, including a strange, vacant doorway, but eerie events begin to unfold and Mortimer's son falls ill. As the supernatural takes hold, Mortimer resolves to do what he must to save his son even as he ventures further into an increasingly horrifying place.