The House of Dies Drear features a house that may or may not be haunted. Some of the characters believe in ghosts; some of them don't. Some aren't sure. Virginia Hamilton, the author, seems to invite each reader to decide. It's like she wants us to weigh in on how we feel about ghosts and other supernatural beings. She also asks you to look at the bad things that can happen when humans accuse other humans of being supernatural creatures.
The House of Dies Drear uses the possibility of the supernatural to build suspense and drive the plot.
The story leaves open the possibility that ghosts could exist.