Rumor has it Truman Capote really wrote most of To Kill A Mockingbird because he was so grateful to Harper Lee for being kind to him as a child. The character of Dill was based on Capote.
That would explain her lack of subsequent books.
I have read that the publishing world knew that Harper Lee wrote most of Capote ‘s famous book “ In Cold Blood “ . She did all the research and he was on a years long bender during the entire period .