The Mystery of Brackenwell Hall by Gillian St. Kevern

Stephen Mereweather, a young invalid, doubts that gloomy Brackenwell Hall can cure him. What chance does country air, no matter how fresh, have against a disease that has baffled the greatest medical mind in London—his own father? Resigned to a long recovery, Stephen encounters instead a cheeky trespasser who takes Stephen’s bad mood as an invitation to be even more obnoxiously cheerful. It’s enough to make anyone reach for the nearest bottle of laudanum…
But Charlie’s inflated self-regard is not the only inexplicable occurrence at Brackenwell. Stephen’s family is plagued by a series of disappearances and deaths. If Stephen wants to avoid a premature death, he’ll need Charlie’s help… But Charlie may prove an even bigger mystery.