Wheatwine:
Case in point, another favorite of mine, Dune by Frank Herbert, one of the 5 best science fiction novels I've ever read. Herbert wrote many sequels to Dune. My only question is, Why?" They got progressively worse until, while I was reading God Emperor of Dune, I kept hoping that the plot to kill off the hero would succeed. (It did, but that didn't stop Frank Herbert from writing still more sequels.)
Well, I thought that Frank Herbert's final two DUNE novels, HERETICS OF DUNE and CHAPTERHOUSE DUNE, were considerably better than #'s 3 (CHILDREN OF DUNE) and 4 (GOD EMPEROR OF DUNE); they're set about 1,500 years after the death of the God Emperor and don't focus on the Atreides family but rather on the struggles of the Bene Gesserit Sisterhood. (Okay, I predictably like strong, dominating females so perhaps that's part of their attraction to me.)
Of course, I felt that DUNE was terrific and I also greatly enjoyed the first, fairly short sequel, DUNE MESSIAH. AFAIC one problem with GOD EMPEROR is that it was written in rather of a vacuum compared to the others--IIRC it was set about 3,500 years after the first trilogy and 1,500 years before the last two books, the only familiar characters from earlier were Leto II (the God Emperor) and of course Duncan Idaho, who kept reappearing as a ghola (a clone created from dead cells) in every book after being killed in DUNE.
To be a bit picayune here, the God Emperor was obviously the title character in GOD EMPEROR, but I'd consider his final Duncan ghola to be the hero of that book if there was one.
While I'm a big fan of Isaac Asimov's original FOUNDATION trilogy and his four robot novels, IMHO the writing quality declined when Asimov tried to connect (plotwise) the different sets of books late in his life.
IMHO Stephen King is an author who, in spite of his extreme popularity, peaked as a writer with his second ('SALEM'S LOT) and third (THE SHINING) novels... --C.K.