The story of Harper Lee's success impresses me greatly.
Of course, we cannot be sure about details, but here's what seemed to happen:
A young woman wrote a novel about a girl just-like-herself who returns to her native town (just-like-the-author's native town) and... And nothing happens. She discovers her relatives and friends are bigots, she argues with them bitterly, then she reconciles with them (because they are her relatives and friends after all). And that's it.
Many good episodes, but nothing extraordinary. Like in the real life. The draft's title was Go Set a Watchman.
Fortunately, the author met a GREAT editor. With her help (and after 2 years of work) she rewrote the whole story:
• She retold her childhood in a funny and endearing way.
• She made her father more heroic and noble than he was, and invented a heroic deed for him (well, took a real fact from his career, moved it 20 years later, dramatized it and showed her dad in the most heroic way)
Result: To Kill a Mockingbird is brilliant.
Of course, one of the reasons of its popularity is that the issue of racial injustice was very important for millions of people (and still is). But it wouldn't help if the book wouldn't be great from the literary point of view.