I do not know the name of any of the books in the series, but I can describe at least one of the covers. Our library had 4 books but there may have been more. All of the books had a deep purple cover. One had a mansion or haunted house on top of a hill or cliff. For some reason murder mansion, Whispering Hill, sticks in my head. I believe there were 2 or 3 kids and one had an odd name. Definitely not the more prolific series, Three Investigators, Hardy Boys, etc. The books were hardcover and not very thick, 70-80 pages and meant for grades 3-6. I hope you can find the series I am looking for.
Back when the Hardy Boys hardcovers were blue and the Nancy Drew ones were yellow, the Bobbsey Twins ones were purple.
The House on the Cliff is actually the second Hardy Boys mystery.
The only series I can remember that always had purple spines was the Bobbsey Twins, like the person above said, although the haunted mansion on a hill doesn’t sound like one of their mysteries. It might be a lesser-known series or one that’s changed its cover art. Do you remember if the kids in the story were boys or girls or some combination of boys and girls?
Maybe the Spotlight Club and/or Brillstone mysteries by Florence Parry Heide and Roxanne Heide?
These sound like the Morgan Bay Mysteries by John and Nancy Rambeau, an 8 book series
Mystery of Morgan Castle
Mystery of the Marble Angel
Mystery of the Midnight Visitor
Mystery of the Missing Marlin
Mystery of the Musical Ghost
Mystery of Monk’s Island
Mystery of the Marauder’s Gold
I forgot about the Morgan Bay Mysteries! I have one of the books in that series, The Mystery of the Midnight Visitor. Does this look like what you remember?
https://forgottenstoriesweb.wordpress.com/2018/06/29/the-mystery-of-the-midnight-visitor/
The Dana Girls? They were a subset of the Nancy Drew/Hardy Boys series’ but not as popular.
I am not the original poster but I’ve been trying to find a book I remember reading in school decades ago & it sure sounds like what I remember was a Morgan Bay mystery!! The book I remember was a mystery, was thin & hardcover, & everything (cover & illustrations) was purple-ish. I loved the book but have never had even the faintest lead – until now! Thank you so much for solving my mystery!!!
I am the original poster and I think the is the book. Our library only had 4 in the series. I read it in the early 70’s