Not a book, but a good long-read article in the new Texas Monthly. It's the story of a little-known school shooting in Austin in 1978. Bit of a stretch to call it JFK related, however the shooter was the 14-year-old son of LBJ's former press secretary George Christian.
https://www.texasmonthly.com/articles/the-school-shooting-that-austin-forgot/
In 1978, an eighth grader killed his teacher. After 20 months in a psychiatric facility, he was freed. His classmates still wonder: What really happened?