I once read a book about a family that moved into an old Manor house. The family experience ghostly writings on a wall, and if I recall correctly they carry out a written conversation with the author of the wall writings.
The 'ghost' was a character from perhaps the 17th or 18th Century and in his day it was his house, while the family was very modern.
To cut a very long story short (because I have a hazy memory of this book) the twist of the book explains that not only do the family think the author of the writings is a ghost, but the ghost also thinks that they are ghosts/alien/paranormal beings. So both seem to believe that the others are paranormal.
I can't remember what it was called and so I ask the community.
'The Others' seems to ring a bell with me, but I don't know if that is because I have seen the film of the same name that has striking similarity with this book, either way I can't match the book with this name. I would very much like to find this book again!