Long time writing
Having just read what is apparently the 37th Discworld novel, I was reminded of the problems of writing a series across a number of years. Aside from continuity problems (which I've touched on before), there's the change in style and viewpoint on the part of both the author and the reader. I've noticed it with Terry Pratchett before, as well as with Stephen King's Dark Tower series, AS Byatt's Frederica Potter novels and Anthony Trollope's Barchester novels.
I'm no longer as desperate to read the new Terry Pratchett or Robert Rankin as I once was (though I do still read them all eventually) and I can't decide whether that's due to a change on my part or theirs. I've been reading Discworld novels for twenty years, and Robert Rankin's for about a dozen, and it's pretty safe to say I've changed a bit in the meantime. Jokes that cracked me up at fourteen seem a bit worn and juvenile now, and I do sometimes get the impression that they're doing it because that's what they do. Rankin at least hasn't only written one series, but how could you be Terry Pratchett and write, say, space opera? Stephen King's Dark Tower series seemed to suffer from a waning of enthusiasm, so that to me the final volume felt like it came about because the story needed wrapping up, not because he needed to complete it. I could be wrong about that particular instance (and I apologise to Stephen King if his heart and soul was poured into that novel and I failed to appreciate it. Because of course he'd be reading this...), but I can see how that would happen with a sparse series like that - you start it when you're young, write a few volumes when you can fit them in over the next couple of decades, then you find you're not that interested any more but other people are and part of it's already out there, you can't leave it unfinished so you tie up the loose ends in some acceptable way, call it done and go back to your main concerns.
Characters change and develop, that's only natural, but sometimes they become someone else, and you can't see any plausible path that got them from their first appearance a dozen books ago to where they are now. Modes of speech, opinions, habits are suddenly at odds with what the reader expects. It's more noticeable for someone who has read the series in concentrated form, discovering it perhaps years after the early volumes were published and catching up in a hurry - the origins are clearer in their mind than in that of the author, for whom book one was half a lifetime ago. Even if the author can get back into the skin of a character, they might not want to, writing instead the character as they would write it from scratch were they to begin today, perhaps with more complexity. But a revised version of the character is not what hooked that reader in the first place; I've given up on a series before when I realised while I was waiting for the next installment that I no longer cared what happened to any of the characters.
It took me six years (with some very long gaps) to write Wasted Years (the one I used to refer to as my serial novel), and while OneMonkey and my friend T read it chapter by chapter over the last couple of years, I wouldn't want anyone else to read it until I've gone back and smoothed it out. I'm sure there are some continuity errors (hopefully not as many as the average Philip K Dick novel) but apart from that my style and ability changed (for the better, thankfully) and it's particularly noticeable where I put the novel aside for a while and wrote other things, honing my craft as it were. I will never understand how Anthony Trollope released novels as serials, writing it from month to month as he went along.