Harper Lee wrote the book. Ironically, she was indeed the personal secretary for Truman Capote.
If you've spent any time in rural Alabama -- Monroeville, Union Springs, Fitzgerald, Hayneville -- you'll recognize the settings. And the author's affinity for them.
They were cousins -- or relatives of some sort.
There's a theory that he actually wrote Mockingbird and she wrote In Cold Blood.
She helped him with the research and probably they read each other's books through before publication.