Yep you described exactly the issue.
I do not have the book on me at this second, so I cannot check who has the copyright, but I am guessing it was her. She long ago died (about 3 years after the book was published). If I recall right, it was published around 1968, so it unfortunately never had a chance of a renewal not happening automatically if I am reading the copyright laws correctly. In fact, it appears it would be 95 years from the publication date before it can go into the public domain.
That is 2063...which means I will be 81 before it goes into the public domain! How asine is that? What sense does it make for a book that probably never had anything close to a second printing, probably sold only a few hundred copies, and was out of print by the time I was born to not be in the public domain until I am 81?
My only reasonable hope is if I could track down my great-great-grandmother's will and hope that it was my great-grandmother (who died in 1996) that was the hier. Because my great-grandmother would have given everything to her 2 children, my grandfather and great-aunt, both of who are still alive.
But given my luck, I doubt it would be that "simple."