Little Free Library Nancy Drew find. This one is also from 1982. Nancy stays at home for this mystery and has help from Bess and George. It takes place in the fall, so maybe Ned and the other boys are off at college.
Carson Drew introduces Nancy to Pierre Michaud, a young French engineer. He has come to the US to try and sell his new computer memory invention (so modern!) and to River Heights because he found an old letter from a resident, Louise Duval, in his grandfather's papers. The letter said that Louise had made a discovery regarding one of her ancestors that would be important to herself and to M Michaud and she was planning to come to France once she had all the facts. Now Pierre wanted to know more.
Nancy discovers Louise passed away years ago but has a nephew and great niece, Lisa Thorpe, still in town. Lisa and Pierre seem to get along, but her father detests him for some reason. When Nancy meets Pierre, she notices they're being watched. Of course, this only encourages her, and she eventually finds other people who knew Louise and lead her to an old trunk of things that belonged to the ancestor, Yvette Duval. From there, everything comes together in a convoluted plot worthy of Hitchcock - including a MacGuffin about a prize-winning artist copying an old painting, ancient French royalty who somehow still had money in the 1980s, industrial espionage, and the ancestor coming down with amnesia (!?!) while on the run from the French Revolution Terror.
I did enjoy this, even as I laughed out loud at the amnesia.