Question: 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: 
> 
> [Any sufficiently advanced magic is indistinguishable from technology.](https://tvtropes.org/pmwiki/pmwiki.php/Main/SufficientlyAnalyzedMagic)
> 
> 
> 

If you try to make sense of it, it stops being magic.

If that is not a satisfactory explanation for you, remember that on the disc, nothing is constant for too long. Even the stars tend to move and form new constellations, much to the chagrin of astrologers.

Last but not least, we are getting that information from secondhand sources and hearsay. Take the Rite of AshkEnte, for example. It only takes a simple incantation and two cc's of mouse blood, but if you ask the faculty for instructions they will include candles, cauldrons, a circle of eight and more paraphernalia. If you don't know better you may be fooled by wizards' love for power displays and drama.
My interpretation is that in "The Colour of Magic" you don't have to create the spell to use it. You have to "capture" it (or channel it, if you prefer) and that spells have an existence beyond the manifestation of their effect.

It would seem that pronouncing a spell lets it free in the world allowing it to influence it as it is its nature. Powerful spells are often portrayed as having a will of their own in Pratchett's work and spells as a category are often referred to as "unstable" and "dangerous", which could be interpreted as a description of their temper instead of a description of their effects.
By that line of thought it could be that you could "acquire" a spell either by finding it through inspiration or by reading it or by walking a particularly bad patch of wild magic. You don't quite need to "create" it as spells can have an existence of its own (but of course it could be possible, after they have to come from *somewhere*).  

Esk is a skilled magician. As such it would make sense for her to be able to recognize spells almost intuitively. Of course such a thing is rare but on occasions spells (and magic in general) have a way of being attracted to certain individuals, just like Rincewind was able to catalyze and attract magic when he was exposed to it or other magicians at times are shown using magic almost reflexively without actually preparing spells. Esk would probably be able to channel such forces while Rincewind actively tries not to for everyone's sake (but mostly his sake).  

Also there was Sir Pratchett's utter disregard towards the limitations of consistency when he had a fun idea.