Inspired by the "made long after" post.
The first two Foundation books were amazing. The rest of the series, published some 25 years later, has very little of the elegegance and vitality of the originals. Ditto Asimov's Robots books. The author changed quite substantially over the years, and it shows.
I did my Grade 13 English Independent Study Unit (ISU) comparing and contrasting these series to each other and across decades. The differences really shocked my tender 17 year old brain.