I looked into it a good deal but haven't read it.
My understanding is that it is NOT a sequel, but was an earlier draft of "To Kill a Mockingbird", which dealt with Scout as an adult reflecting back on her childhood and that perhaps her father wasn't as he seemed. The editors at the time said "we like the childhood part, go with that" so she rewrote it and we got the famous book.
I do find it highly suspicious that after decades of Lee saying she would never publish another book, and just after her "guardian" sister died, when Lee was in failing health, nearly blind and deaf and possibly suffering from dementia, her lawyer announces that she had "changed her mind" and wanted to publish this early draft now. And no one has been allowed to ask her for confirmation on this since all communication goes through said lawyer.
So out of principle I refused to read it.