I read this book late 90’s / early 2000’s (1998-2002), but the book could have been written any time in the 90’s. I know it was a mystery book aimed at 10-15 year olds, the main character was a boy who was about 13. (there may have been a female character too) The boy explores / investigates this abandoned gothic mansion, coming across clues as he does so. One of the clues is the word “Boss Stone” which eventually he realizes is the architecture term (“knob or protrusion of stone or wood”). He realizes that there is a “boss stone” located on the ceiling in the main room of the mansion, and once he gets access to it there is something hidden behind it. The only other part i remember is that the building is haunted / guarded by this cloaked figure that has a tentacle.