To quote a review on Amazon.co.uk, it is ‘the authorised 50th-anniversary sequel to The Day of the Triffids (1951) by John Wyndham, that classic SF nightmare which gave our language the word "triffid".’
There are a large number of such ‘authorised sequels’ in existence. Gone with the Wind sequels Scarlett and Rhett Butler's People, Peter Pan in Scarlet, The Dragon Lensman, the James Bond continuations by James Gardner and others, and many more.
There are a lot of follow-ups to Robert E. Howard’s Conan stories written by other authors. First came L. Sprague de Camp and Lin Carter’s stories that were based on uncompleted drafts of stories by Howard himself, then they wrote original material, and then everyone else seemed to get in on the act. Examples include Robert Jordan’s abominable Chronicles of Conan, all the Roy Thomas Conan graphic novels that were his creations rather than re-tellings of the Robert E. Howard originals, Harry Turtledove’s new Conan of Venarium, and Conan and the Grim Grey God by Sean A. Moore which is notable because the only review on Amazon.co.uk is the author’s own (beginning My favorite of the Conan novels I've written thus far - I rather doubt that anyone else will describe it as their favourite anything!).
They’re fanfic. Some are reasonable, many are crap, but they’re all fanfic; unless the contracted author didn’t really like the original material and is writing solely for the money, which usually produces extremely flat and tedious results.
Fanfic, then, to the Establishment, is something that is of universally poor quality and to be derided – unless they are making money out of it!
I noticed something else odd today. There are no Conan crossovers at all on 'Twisting the Hellmouth'. I wonder why?