In addition to the others mentioned there's
James, Percival Everett's version from the perspective of Jim
Huck Out West, by Robert Coover, a revisionist take where Tom Sawyer sells Jim to buy a pair of boots, Becky Thatcher ends up in a brothel, and Huck has some solo adventures before meeting up with Tom again, who's now a shady lawyer running for Congress.
spoiler
Rewriting Twain, John Seelye delivered a raucous R-rated adaptation in The True Adventures of Huckleberry Finn (1970), notably discarding Twain’s comical ending for a more tragic conclusion depicting a manacled Jim drowning in a last-ditch swim for freedom. Going in a new direction, Greg Matthews took up Twain’s original cue for a westward turn in The Further Adventures of Huckleberry Finn (1983). Matthews’s version received mixed reviews — among them this jewel: “The result is sometimes cute, but more typically unreadable […] no Twain wit or wisdom […] like Early American–style furniture made of plastic laminate.” Two more indirect adaptations/extensions have come in the form of Nancy Rawles’s My Jim (2005), a moving story narrated by Jim’s enslaved wife Sadie, and Jon Clinch’s Finn: A Novel (2007), which takes on the unenviable task of telling the story of Huck’s father, Pap. Whatever the outcomes of critical debates or successes of the respective novels, imagining Huck’s adventures after his reluctant coming of age and his decision to risk hell in stealing Jim out of slavery has been both fruitful and fraught-filled.