Fantasy novels are growing up. As in they are getting longer and longer.
When J R R Tolkien published Lord of the Rings, he did it in three books. This has inspired many an author who followed in his footsteps to do the same. In fact it’s practically a requirement for epic, high fantasy. However fantasy authors have been trying to out do Tolkien and make their series even larger. This might not be a good thing. It might do things like make your readers loose interest. And result in bad writing.
The word trilogy means three books in the fantasy novel lingo. Writers like Mercedes Lackey, who wrote multiple books set in the same world, did it in trilogies. Others like Terry Brooks went a different route. He wrote most of his Shannara books as stand alone works. None of them dragged the same plot through ten (massive) books.
Because that is exactly what Robert Jordan and Terry Goodkind have done. And this didn’t improve their work, at all. Jordan’s Wheel of Time was great for the first three books, after that it became a maze you had to slog through. The Sword of Truth series by Terry Goodkind had a great first book, and that was all. These series have graduated from trilogy to decalogue. And while I’m sure it made their publishers very happy, it was not good writing.
The extra books aren’t even that important for driving the plot. Most of the space in the Wheel of time books is taken up by fluff. Little interactions by a small army of minor characters. Along with a remarkable number of female characters getting stripped down to their shifts and getting spanked. With this many characters spread out over this many books, it’s remarkably hard to keep track of everyone and what is happening. Especially when the books were published so far apart. As for the Sword of Truth, the plot advanced so slowly it was crawling. And it had so much content that was pure filler. If these shed their extra baggage and got on with the plot, they would have limited to three or four books quite easily.
The current fan favorite, The Song of Ice and Fire, is an estimated seven books long. And while the author had managed to keep the story engaging and the plot moving, it too is starting to show a little extra baggage.
As for other authors, though most of them aren’t trying for the ten book series, their stories are getting longer. At least four books to finish the series, of not more. And these are not small books.
While I feel that a four book series is ok (Calling them a quantaloy is going to be awkward though), anything longer than that is too much. This is just my humble opinion, but I can back it up.
Think about it. Fantasy books are published at least a year apart. If your series has a dozen books, will it keep your readers interested? Just how long is your plot? Just how detailed is your world? How much of it are you showing? Do you have to tell every little side story right here in these books? You can always publish another book set in the same world later on if you want to tell more stories. You can even write another trilogy just for that. Remember, J R R Tolkien had an incredibly detailed world and a complicated story, but he told it in three books. He did this by keeps the extra plots and minor characters away from the main story. Tolkien never tells the story of second battle of the Lonely Mountain during the War of the Ring, or how the elves of Lorien withstood the assault by Dol Guldur and went on to conquer it after the fall of Sauron. These events were simply mentioned. He focused on the main plot.
I believe that this is the way to come up with an engaging series of books.