If you feel like you've noticed more and more science fiction books labeled "Book Seventeen of the Outer Explosions saga," you're not imagining things. A plague of sequels has overtaken science fiction books, judging from a statistical study in StrangeHorizons.
Contributor Valentin D. Ivanov, who is our new hero, went through the "notable books" feature and book reviews in Locus Magazine since 1998, and created a statistical analysis of the trends. The trend in sequels is especially fascinating: It looks like there was a huge spike in sequels from 1998 to 2001, and since then the number of sequels has grown at the same rate as all other books.
Ivanov notes that "there are only two fourth or fifth volumes in 1998/9, eleven in 2001/2, and forty-one in 2007, corresponding to a nearly 20-fold increase." In other words, we're not just talking about trilogies, we're talking about a massive increase in series spanning four or more volumes. (And I believe he's separating out media tie-in books, so we're not talking about Star Wars novels here.)
A few other trends Ivanov notices: There's the rise in prominence of fantasy books that you've already noticed elsewhere, except now it has statistical validity. The number of fantasy books being reviewed in Locus went up by 26.7 books per year, as opposed to 11.5 for science fiction. Also, the number of multi-author anthologies went down slightly, despite a number of anthology series making waves in recent years. And there's an overall increase in the number of books being published generally. Finally, it's interesting to see which publishers get their books reviewed in Locus most often — Tor has had 210 books reviewed in the past decade or so, more than double the next biggest publisher, Harper Collins.