You have nothing to lose by reporting it. The fact that Carlene took violin lessons and that this man was a violinist who may have given lessons is enough to keep me from dismissing this as a coincidence. And he may have lived in FL during his older years but that doesn't mean he couldn't have lived in GA during Carlene's lifetime there. I live in a southern state, not as far south as GA, and a lot of people from my state move to FL when they retire so I think there's a chance your neighbor could have moved to FL from GA in his later years.
On the one hand I know people often donate old books and belongings when a person has passed away, but on the other hand it's hard for me to imagine the family of a girl who went missing at 16 giving away a book she'd written her name in not once but twice. Her parents are deceased but it was only a few years ago that her mother passed. If I'd been her mother I could not have donated or thrown away a book she'd written her name in. The book meant enough to Carlene to put her name in the front and the back and that probably means she intended to keep the book to reread at a later date. She must have liked it and that would make it a sentimental item for her family, I would think. I still have books of my late mother's that she wrote her name in because I just can't part with them due to her signature being in them even though she's been gone a very long time now.
I think you are doing the right thing to report it, because what a strange and surprising thing this is, to be given a book with the name of a missing girl inside it. There's a possibility this could bring some answers about her fate. But if there's no connection between Carlene and this man, at least you've done what you could.
This is my #1 top case I'd like to see solved. I don't know why but from the moment I first heard of Carlene's case I've never stopped thinking about it. Of all the missing persons cases I've ever read, no matter how bizarre the circumstances of some of them are, Carlene's case is the one I most want answers to.