Terry Pratchett's Discworld: is there an inconsistency in the treatment of magic?

In The Colour of Magic, we learn that:

"Can't tell you. Don't really want to talk about it. But frankly," he sighed , "no spells are much good. It takes three months to commit even a simple one to memory, and then once you’ve used it, pow it's gone. that's what's so stupid about the whole magic thing, You know. You spend twenty years learning the spell that makes nude virgins appear in your bedroom, and then you're so poisoned by quicksilver fumes and half-blind from reading old grimoires that you can't remember what happens next."

So clearly: a spell is something you need to create, and you can use but once. And it's something utterly useless as a consequence.

In Equal Rites, Esk is able to do MANY spells without creating any of them. And afterwards, it seems that The Colour of Magic description of how magic works never holds...

Does Rincewind lie? Is there an explanation I missed out?

Answer

The best explanation is that the first two Discworld aren't really "Discworld" books as much as they are "Rincewind" books. They're much more aimed at being parody of a large number of fantasy genres than they are designed to be a unique work that starts a brand new franchise.

There's tons of inconsistencies between the early books and the later. In the first pair of books, for example, Death only showing up at the death of Wizards, and in Rincewind's case, he only gets Scrofula. In (IIRC) Equal Rites, (EDIT - sorry, Weatherwax is from Light Fantastic - it's Archchancellor Cutangle in Equal Rites. The Management apologizes for the inaccuracy.) we meet Archchancellor Weatherwax of Unseen University, and the rules of magic seem based in balance - the Archchancellor wants to fly up to another level of the hall, so he casts a spell that makes a bit of concrete of equal weight fall downwards. He's replaced by Ridcully in later books, especially after Granny Weatherwax became a major character. (There's a callback to that in a later book, where Ridcully mentioned that they'd had an Archchancellor Weatherwax, and Granny offhanded mentioned that he was a distant relation and didn't know him.) He's even chosen to bring back characters that proved popular, like Gaspode the dog, who appeared to have a definitive (albeit somewhat happy) end of his tale in Moving Pictures.

As with any number of series by many creators, the rules and continuity don't really settle down and become what we really see as the proper series for a book or three. In the Destroyer series, Master Chiun is described as having been hired to teach Remo Williams (shudder) Karate. The Sun Source of all Martial arts, Sinanju, is only mentioned in book three, Chinese Puzzle. There's also the Kids in the Hall bit about the Doors - "Their third album is really their first - it's what we call...'The Departure Point'."

Some writers choose to go back and "fix" the earlier books to more correctly reflect the later-established continuity, some prefer to let them stand, and in both cases, the fans may or may not choose to create headcanon to explain the "errors".

But if you need to find a real explanation, all of the inconsistencies can be blamed on Quantum.