I remember as a kid (probably late '70s) reading a mystery where the protagonists, probably 2+ boys, but I don't recall, interviewed the remaining residents of an old plantation for clues. One of them, an elderly woman (a centenarian I think), had been a slave on the plantation before the Civil War.
It struck me at the time that - wow - of course, former slaves would still be living into the 1900s, and I wonder... but of course by the time I was old enough to read it they were all gone (the last known former slave died in 1971). The story must have been written in the 30s or 40s. It might have been a Hardy Boys book or something, but I can't seem to find it anywhere. Does anyone have any idea?