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.