Series that got better over time:
Harry Potter
I maintain Book #5 is the best of the series with an extreme quality drop-off after that point, perfectly captured by 200+ pages of camping in Book #7.
As far as Sci-Fi authors, no one's giving any love to this guy?
Many of his books/stories are easily classifiable as science fiction (Sirens of Titan being a prime example) and many of his books take place in alternate timelines (Galapagos).
Plus, he just published another posthumous collection.