I've just sent the first draft of Hope and the Clever Man, the second Gryphon Clerks book, to my lovely and talented beta readers. This is the culmination of a five-month-long writing process; a few of the chapters were recycled, having been cut from an earlier work, but most of them are new.
It's also the start of an editing process that I expect to last two or three months. First the beta read, so I can get other eyes on the text for the first time and find out what's missing, or not working as it should. Then an intensive edit from me, in multiple passes, strengthening and expanding. Finally, two passes through with my wonderful development editor, Kathleen Dale, who helped make Realmgolds a much better book than the one I initially gave her.
Hope is about a third longer than Realmgolds (80,000 words, at the moment, versus 62,000), following the usual law of series by which the later books get longer. It overlaps Realmgolds in time, which makes it tricky to avoid spoilers, but I think I've managed it. There are a couple of characters who appear in both books: the redoubtable Realmgold Victory strides in and orders people about several times, and the venerable Master-Magus is back too. There's a brief cameo from Leading Clerk Grace Carter, just for fun, and the Realmgold's clever man, mentioned a couple of times in Realmgolds, has, obviously, an important role. Otherwise, the characters are all new.
The idea is that either book acts as a point of entry for the series; you don't have to read one before the other. I want to carry on that approach in future, creating a series more like Terry Pratchett's Discworld than, say, The Wheel of Time. WoT lost me as a reader years ago when the books started to be spaced further apart, with the promised resolution nowhere in sight and a huge, complicated cast winding round each other before disappearing up their own backstories.
There will be recurring characters in the Gryphon Clerks, recurring groups of characters, even. I love my characters, so do my readers, and I enjoy leaving Easter eggs like a brief appearance in one book by a key character in another. You won't have to read from the first book in order to understand what's going on, though. (You also won't have to read to the last book to understand what's going on, which means that, while hopefully the fans will still stay eager for the next one, at least there'll be a story that finishes in less than 20 or 30 years.)
Not having to start with Book One is important, because, while most of the reviews I've got for Realmgolds are four stars, even my greatest fans acknowledge that it has its flaws. I'm aware of that too. The first book of a series is frequently the weakest, or one of the weakest, even in series that go on to be hugely successful.
Look at Jim Butcher's Dresden Files. Personally, I like Storm Front, having been into Jim Butcher since before he was cool, but a lot of people don't, and even I freely acknowledge that the later books are much better. Fortunately, Butcher makes most of the early books, at least, reasonably self-contained, and does a good "who are these people and why are they angry" catch-up at the start of each new one, so you can read some of them out of order or even skip some without missing too much. At the same time, he keeps an overall arc going, which he's said is going to take about 20 books or more to complete. (I don't plan on doing that, partly because I'm not nearly the outliner he is.)
Pratchett himself didn't really hit his stride until Mort (in my opinion), maybe even Wyrd Sisters, and those are the fourth and sixth books in the Discworld series. I'm a huge fan of the Discworld, but my top favourites start about 10 years after the publication of the first book, around Men at Arms, which is book 15. Not to say that there aren't excellent books before that, but Men at Arms started a really strong run of writing which was deeper, more thoughtful, and consistently good, a run which lasted (again, my opinion) for another dozen years and 20 more books, finally broken, briefly, by Wintersmith. If everyone had to read 15 books before they started to be consistently good, I'm not sure the Discworld would have enjoyed the success it has. The multiple entry points ensure that they don't have to.
Another approach to series which avoids the one-story-in-12-books-released-over-30-years issue is to write self-contained stories in smaller numbers of books, with these mini-series then potentially having some kind of link to each other. Trilogies, of course, are popular, especially in fantasy. Lindsay Buroker has recently concluded her Emperor's Edge series, which is seven books, plus two other loosely connected novels set in a different part of the same world and some interstitial short stories. Because she writes fast, and they're short, the whole series has wrapped up in three or four years. She now has the option of writing other stories in that world, or even with those characters, while also having a completed series.
When you're writing fantasy or SF, worldbuilding takes a good deal of your time unless you're using a vanilla off-the-shelf setting (which is usually pretty dull), and there's definitely an advantage to being able to re-use it. Even Brandon Sanderson, who has the most insane ability to come up with new magic systems, writes more than one book in most of his settings. I created the foundations of the Gryphon Clerks setting over several years, and there is certainly plenty of space to tell stories in (we haven't even glimpsed what's over to the east yet).
Besides that, of course, it's becoming received wisdom that a series will hook more readers than a standalone novel, for much the same reason that it's easier to sell a novel than a novella, and easier to sell a novella than a short story. Readers, like writers, put a certain amount of investment into a setting and a group of characters, and if they like them, they want more of the same, only different.
Telling loosely connected stories lets me indulge my taste for novelty while also avoiding the work of building a new world each time, and hopefully will keep the readers hooked as well, while allowing them to skip a book or two if it isn't to their taste.