At long last, I have now read the Villiers novels of Alexei Panshin. I'm tempted to say "Villiers Trilogy"; it's pretty clear that the series can never be continued, simply because Panshin is no longer the author who wrote the first three, and the time is not the time that made the first 3 possible. (They were written in 1968-9.)
I call them novels, but by modern publishing standards they are not. The three of them combined would not approach the total length of any of the last 4 Harry Potter books, or a recent David Weber doorstop. Star Well and Masque World are both 156 pages in my mass-market paperbacks; The Thurb Revolution (the middle book) stretches to 214. No publisher would touch a novel at that length today; you can't sell it for enough money. At best, the three would have been packaged into a disguised omnibus, marketed as a single novel from the start.
I will say up front that I have an extreme weakness for "science fiction of manners", and the Villiers novels always make the short list of notable works in this category. I was charmed by the digressions of the omniscient author, the modesty of the ambitions of the plots, and the ubiquitous touches of gentle irony and outright farce. Not every attempt was successful, but the vast majority of them were. I surely wish someone were still writing such books, be it Panshin or someone else.
For better or worse, I think the revelation at the end of book three makes not so much a cliff-hanger, as a catharsis. That may be making a virtue of necessity, but it is certainly not as unsatisfactory a place to stop as (say) Patrick O'Brian left us with. For small favors, we give thanks...
Professor of Story Problems, emeritus
Rationalist with sombrero