I'm in two minds about this, given some of the stories suggesting that Harper Lee never wanted her book published and is not of a mental state to make decisions any more...
I thought she had tried to publish this one first, but they didn't want it published before Mockingbird? IDK though, I haven't really been following this story.
Not entirely convinced about this either -- sequels with many years in between them can go horribly wrong. As an example, consider Tales from Watership Down, published around 25 years after Watership Down. It was a disjointed collection of short stories of uneven quality which I felt only diminished the original masterpiece.
Maybe it's an extension of 'The Long Goodbye'?