(or "The Amazing Transforming Meta-Engine!")
"The Colour of Magic", the first Discworld novel, is far from author Terry Pratchett's best work. In fact, it could be the weakest of all the Discworld stories; it's essentially a rambling, shaggy-dog tale of an almost terminally naive tourist wandering through a stereotypical fantasy kingdom, upsetting numerous apple-carts with the way he treats the life-and-death struggles of its heroes and villains as entertainment. That's not to say that it doesn't have its good bits; it is, after all, Terry Pratchett we're talking about here. But it's not particularly focused, and it's not very subtle in its satire of the fantasy genre.
Which is why it's also the clearest example of just how the Discworld storytelling engine works.
The parodic characters that appear in "Colour" are pretty broadly sketched, and easily recognizable as cousins to their non-humorous counterparts--Hrun is Conan, Bravd and the Weasel are Fafhrd and the Gray Mouser, and Bel-Shamharoth is Cthulhu with the serial numbers filed off. This means that it's easy to recognize the book as what it is, a commentary on the tropes of fantasy novels (general and specific.) Pratchett is writing metafiction, drawing inspiration from his predecessors even as he satirizes the conventions of the genre.
To which all Pratchett fans say, "Duh!" But Pratchett continues this trend for quite some time, even as he gets better at disguising it and more creative in his satire. Cohen, for example, is as much a Conan parody as Hrun, but instead of simply being another big dumb barbarian, he's a fully realized and poignant character with real depth. But he's still a parody of a fantasy trope, just as Granny Weatherwax satirizes the conventions both of the "young apprentice" genre of fantasy novels and the traditional notions of "wicked witches" from fairy tales and popular culture. Even as the cast of the series expands (Lawrence Watt-Evans identifies eight separate sub-series in his book "The Turtle Moves"), the metafiction aspect remains a constant.
But significantly, starting roughly around the time of "Wyrd Sisters", he's no longer just satirizing fantasy, he's drawing on reality for inspiration as well. Hwel, the dwarvish playwright, is used as a lens to look at Shakespeare and the theater in general (and cinema as well, in a few memorable gags.) The Ephebian philosophers in "Pyramids" twist and warp the ancient Greeks (to delightful effect.) Pratchett is still writing satire, but he's gradually moving away from metafiction into satire of the real world. "Moving Pictures" and "Soul Music" are the most obvious and direct examples of this, but all the Discworld novels take on the flavor of a world we recognize.
But the real world is decidedly unlike fantasy kingdoms in one major aspect. (At least one major aspect, that is. Obviously, the real world doesn't have vampires, werewolves, dwarves, trolls, and magic...although curiously, this is where magic, in the showy flashy sense, really starts to vanish from the series.) A fantasy story is usually about the restoration of the old order--once there was a long, static period when things were good, things are now changing and that's bad, the hero brings back the true king or the magic talisman or whatever, and the change is undone. Let me just re-emphasize that bit. The change is undone.
In "Moving Pictures" and "Soul Music", that's exactly what happens. With a magical zap, the wild ideas that infested the Discworld are forced out, and everyone feels slightly sheepish and wonders why they were so obsessed with the clicks or Music With Rocks In. (It's significant that the danger in "Moving Pictures" is that the Discworld is such a thin construct that it could be torn apart if people stop believing in it. Too much reality makes the cracks in the fantasy show.) Pratchett is using the Discworld to comment on reality, but the Disc itself is still recognizably a fantasy world. (In the sense of being static and unchanging, that is. It's always a recognizably fantasy world in the sense that it's flat and carried on the backs of four elephants which are, in turn, carried on the back of a giant turtle.)
But reality, that has a way of changing constantly. Sometimes for the worse, but sometimes for the better. While people might like to remember a long, static period when things were good (and write that nostalgia into fantasy stories which then become meat for satirists), the fact is that the past wasn't as static as you remember, and probably not as good, either. A series that's genuinely satirizing the real world, instead of just borrowing elements of it, is going to have to change.
And so, the storytelling engine of the Discworld, the elements of it that help Pratchett come up with story ideas, that has to become about change instead of stagnation. The first novel that undeniably ends with the world in a different place than it began is "Small Gods", a novel that uses the Church of Om to comment on the ways that religion has modernized to fit into a newer, more enlightened secular world. In addition to being frankly brilliant, it is a novel that doesn't retreat from the ideas it expresses. The Omnian Church after "Small Gods" is a different church, inspiring different ideas in later novels.
But "Small Gods" also takes place some distance away from the main events of the Discworld. It's a big shift, but it's one that's easy to ignore when it doesn't affect Vimes, Granny Weatherwax, Rincewind or even Death to any significant degree. No, the novel that definitively states, "The world is changing and this is now the well from which inspiration is drawn," is "The Truth".
Watt-Evans dismisses "The Truth" pretty casually in "The Turtle Moves", lumping it in the same pile as "Moving Pictures" and "Soul Music" (with "newspapers" taking the place of "movies" and "rock music".) With all due respect to an author whose work I enjoy, this misses the biggest point of the book--this isn't a wild idea, summoned by magic and prone to rip a hole in the fabric of the Discworld. This is a printing press, made by mundane hands and inspired by good old-fashioned intelligence. (Pratchett even comments on his own tendencies at one point, as the Patrician asks a series of weary questions in an effort to make sure that the whole enterprise isn't going to destroy the world.) The notion of change, of modernization really crystalizes here and becomes the engine for the series at just about every point after that.
Unfortunately, Terry Pratchett's tragic diagnosis of Alzheimer's means that the Discworld series will be winding down soon. Although he is continuing to write for as long as he can, the disease will eventually take its toll, and I think I speak for just about all fans of the series when I fervently hope that nobody will attempt to fill his shoes just for the sake of keeping the franchise going. So after perhaps forty novels, a half dozen short stories, and numerous ancillary books and other materials, the Discworld's story will end. But that body of work is a testament to just how well the Discworld's storytelling engine works, as one of the best satirists in literature today turned his observations on the world into stories time and time again.