Some books draw their inspiration from other people's work or from real-life events, and then invent a corresponding story -- something that might have happened. Here are a few:
Daisy Buchannan's Daughter -- the story of what became of the young Pamela Buchannan from The Great Gatsby.
March -- what was going on with the March sisters' father while the story of Little Women was unfolding?
Then I came across a few that use the Kennedy assassination, and its characters, to create new fiction:
11/22/62, Stephen King
Libra, Don DeLillo
Oswald's Tale, Norman Mailer
As it turns out, all of the above are authors whom I have always wanted to read, but never have. Time to crack those spines.