Red pencil

Wow, there is so much wrong with this copy.  First, you don't have a sequel OF something; you have a sequel TO something.  Second, as it was "written before the best-selling novel," it can't be a sequel at all; if anything, To Kill a Mockingbird would be a prequel to Go Set a Watchman.  And finally, neither sequel nor prequel is appropriate terminology here, since Lee wrote Go Set a Watchman and her publisher suggested she rework the flashback scenes into their own separate novel of Scout's childhood, which would make this a first draft of To Kill a Mockingbird.