Earlier quoted context omitted.
I've read the Frank Herbert Dune books back-to-back several times and love them all, but I've never tried his son's additions to the series because I've read a few people saying they're not worth the time. Do you agree?
They're quite bad, and I doubt very much they are based on Frank Herbert's "found notes" in any way. If they are, Brian Herbert should publish those notes, like Christopher Tolkien did. Lower quality writing, and not thoughtful, and the story arc they take things in seems much cruder than what Frank Herbert would have had in mind.
Consider Variable Star by Spider Robinson which is based on 7 surviving pages of 8 pages of notes written in 1955 by Robert A. Heinlein.
http://www.spiderrobinson.com/reilly.html
> So I went home, and received a copy of Robert’s outline and notes, and loved them, and wrote two sample chapters and a proposal and a title (Robert had put down seven possible titles, but even he didn’t like any of them much), and they were all approved by Art Dula, and in the fullness of time the book, to be known as “ROBERT A. HEINLEIN’S VARIABLE STAR by Spider Robinson,” sold to Tor for the proverbial six figures.
> Since then, little dividends of joy keep coming in, like the receding ripples of pleasure that accompany a truly great orgasm--and sometimes, if you’re lucky, signal that it’s about to become a multiple. For example, Art Dula moved me to tears by sending me, out of the blue, Robert’s desk dictionary, heavily used and carefully repaired--Robert Heinlein’s personal box of words. He filled out the package with a pound of authentic Jamaica Blue Mountain. Similarly, sweet Amy sent me a set of her grandfather’s cufflinks to wear as I type VARIABLE STAR, and Jeanne a few pieces of her grandmother’s jewelry to wear for me when I stumble from the typewriter. I feel supported and encouraged by the whole Heinlein family and legacy. That makes me the luckiest writer alive. And one of the luckiest readers.
When writing it, he had the same "these are the words that you are to use while writing as Heinlein."
One of the reviews http://www.spiderrobinson.com/variablefans.html
> This novel should be the example held forth when writers collaborate. Spider has perfectly captured the pacing, feel and stature of a Robert Heinlein story while retaining his own identity. It doesn’t feel like someone trying to “write like Heinlein”, but like someone who’s read every book the man’s written so many times it’s second nature.
https://bookshop.org/p/books/variable-star-robert-a-heinlein...
> "Completing a book from notes by a dead author is almost always a mistake. But Robert A. Heinlein apparently isn't really dead. He was obviously standing at the side of Spider Robinson as he wrote this book, guiding his hand. Variable Star will delight the fans of the greatest science fiction writer who ever lived, and at the same time, stays true to Spider's passionate themes of optimism, kindness, and humanity's future among the stars." --John Varley, Hugo and Nebula Award-winning author of The Persistence of Vision and Steel Beach
This is the way to do continuations - not fan fiction but, for lack of a better word, channeling the spirit of the original author so that you can become them and use the same style rather than imposing your own style on top of their world.