More and more books are being sold as a complete package. This means that many books are designed so they can be written as an endless series and have dozens of different products spun off of them. The people who do this are called "packagers." I am not particularly fond of this. It seems to stunt creativity.
First you have a book or movie, then it books a comic books, an action figure, a television show, then many other products. I sometimes wonder why they continued with Star Trek Deep Space Nine, after creating a very nice original series.
When I go to my local library, there are more packaged fantasy and science fiction novels than original novels in the fantasy and science fiction section. You can get Warhammer 40K, Dragonlance, Star Wars, Star Trek, Deathlands, and other series, but not as many original stories.
The majority of these are atrociously written. Serials are written from guidelines, the characters, setting, and actions must fit, or the next author can't continue the series with consistency. The fans want complete consistency and very little originality. This is the place for new authors to cut their teeth. Because the books fit a very narrow focus they are much easier to write. Romance novels follow the same pattern, a very specific outline for each specific series.
There are a few good authors who started this way. Peter David started by writing Star Trek, David Gerrold wrote quite a few Star Trek novels, and Marion Zimmer Bradley started her career as a gothic romance novelist.
I think this has gotten out of hand. It is the urge to monetize everything and milk a novel or book for every bit of money it can make. If a book doesn't start out as a serial, it is often turned into a very long series. There were three books in the Lord of the Rings, a good and proper number. Now we see things like Robert Jordan's Wheel of Time series being turned into an endless story. Some people find comfort in bestselling serial fantasy novelists like R.A. Salvatore, I find it stultifying.
Some authors write well enough that they can pull this off. The Sherlock Holmes pastiches and Lovecraft pastiches are often quite enjoyable. When I say pastiche, I am referring to novels written about a character after an author dies.
Some serials have even become pulp classics like the Shadow, or Flash Gordon, but for the most part these are long forgotten. They represent the stereotypes and angst of their time and often strike a deep chord in the collective psyche. Occassionally, they are hauled out of the closet and made into a movie like Dick Tracy or the Shadow. Batman never seems to go out of style in the movies.
I congratulate J.K. Rowling on her decision to end the Harry Potter series. It shows a level of maturity in writing. The series will maintain its integrity and originality.
There is money to be made in packaging, collectible action figures, plastic rings, and other things which bring back childhood and extend adolsecence. Now the serials have become collectibles, you have to have the complete run of a million products or you aren't a true Star Wars fan, Star Trek Fan, or comic geek. The cost bankrupts many people and occassionally destroys some lives.
I wish there was less of this. I like a good original novel. Sometimes I would like a novel to end. It seems that if you don't kill the main character, the publisher will ask writers to write another novel based on the same character. I am glad the Walter Mosley killed off Easy Rawlins in his latest novel Blonde Faith, it was a good and appropriate ending for an excellent detective. Now, Walter Mosley can concentrate on writing other things.