I'm finally getting around to finishing the Skylark series! Even though Skylark of Valeron had a pretty definitive ending back in 1934, apparently lots of readers, including Frederik Pohl, wanted more Skylark, and in 1965, Doc Smith agreed. He serialized the new book, Skylark DuQuesne, in IF Worlds of Science Fiction and collected into a book in 1966.
It was Smith's last book before he passed away.
Like a lot of Smith's stuff, the book starts with a crazy revelation--the method Seaton and company used to keep DuQuesne and the bodiless intelligences out of trouble turns out to have a fatal flaw, and probably won't last very long. This kind of reframing or even retconning is a pretty common element in Smith's sequels, so it wasn't too jarring. In fact, the reason given and the logic behind it is subtle but surprisingly sound.
