Twain's original is "A Connecticut Yankee in King Arthur's Court." Never heard of any sequels by him or anyone else but not surprised they exist. It's been parodied and used as inspiration everywhere from Buggs Bunny to Black Knight. Even, IIRC, Stargate SG-1 or some similar show in the late 90s used it. Oh, and MacGuyver appeared to in a VERY weird episode (that I only saw the last 20 minutes of, making it even weirder).
And for other examples of shared universes:
The Thieves' World series - IIRC three independent novels (believe I read one) and at least eight (I only read four) short story collections.
The Marvel Universe
The DC Universe
The concept works best with a single editor creating the overall setting and maybe tossing in a short story or a scenario that can impact one or more stories but mostly just making sure there's a connecting logic, even if events don't always match up.