A Wizard of Earthsea
How we cite our quotes:
"Your tongue will be stilled until I choose to unbind it, and even then, though you can speak, you will not be able to speak the word I teach you where another person can hear it. We must keep the secrets of our craft." (1.12)
Ged's aunt teaches him magic, but she also wants to make sure that he doesn't give away her secrets. After all, if everyone knew what she knew, everyone would be calling goats to them. (We would for sure.) It's funny that the way he practices his power (speech) is also what his aunt tries to take away here.
The Hardic tongue of the Archipelago, though it has no more magic power in it than any other tongue of men, has its roots in the Old Speech, that language in which things are named with their true names … (2.18)
Here Le Guin lays out some of the rules for how magic works in this universe – it works with true names. What does this say about all the other tongues of men? Like, if there's a language in which things are truly named, does that mean all other languages are untrue or false in some way?
For magic consists in this, the true naming of a thing. (3.68)
Kurremkarmerruk explains this to Ged, as well as going on to explain that there are lots of names that people will never know. The idea of true names is a common idea in some cultures, as Le Guin mentions, but let's be honest: the idea that words are magical must be really appealing to an author.