When I was in 5th grade, my teacher did something similar. I know she got the books for free, but she still took the time to sort them and give each one of us a book that she thought we'd enjoy, rather than handing them out randomly. She also wrote a quick note on a Post-it telling us why that particular book made her think of us. It was a great gesture!
I think NN should be deleted. If she asks, the OP can tell her that her negative comments were becoming hassles. Or whatever she deems fit. I don't think an explanation is due at the time of defriending.
On a related topic: The book my teacher gave me was something about dragons, and the main character's name was Jakkin. If anyone knows the title of this book, I would love to know so I can go out and rebuy it!! I loved that book and it was lost in a move a few years ago!