The public library I visited as a child had a young adult book that scared me half to death. Here's what I remember:
- The cover art was in the style of Edward Gorey (black and white line art)
- The setting was Victorian or thereabouts (no electricity, motorcars either non-existant or unusual)
- The protagonist was a child isolated from their immediate family: either an orphan, or visiting distant relatives for the summer, or something like that
- There was a big, mostly abandoned building (a museum? a mansion with an abandoned wing?)
- This building had an invisible monster - maybe the word "ghoul" was used - that would prey on anyone in the building at night
- You would know if the ghoul was close because candle flames would turn blue
- The child ends up needing to go to the building at night, carrying a candle, and the flame does indeed turn blue at one point
I think I tried to read this book on two different occasions and never managed to finish it. Besides overcoming some childhood trauma, I'm also interested in the "sword turns blue when Orcs are near" / "candle flame turns blue when the ghoul is near" trope.
This was a small public library in Canada in the late 1980s.