Simple, Realistic
Magic is a way of life for many of the characters in Sabriel. There are no holy cow moments of astonishment or whimsy when we first see the Old Kingdom—nope, it's snowy, dangerous, and lonely.
This system of magic is deeply important, and the author treats it as such, writing about necromantic bells, Charter symbols, and binding spells as if they're completely real. And this realism creates a truly immersive fantasy world, adding to the sense of life-or-death peril in Sabriel's adventure. No silly spellcasting or amusing place names here—we've got no choice but to take magic seriously.
We first see Sabriel cast a protective spell atop Cloven Crest, and she seems just like a kid about to take an exam, except she's trying to make magic:
Sabriel bit her lower lip till it hurt and her hands, almost unconsciously, fidgeted, half-drawing Charter spells in nervousness and fear. (5.13)
Because Sabriel's nervousness seems real, the magic seems real, too. And because Nix keeps the language simple—we're not caught up in the language of an elaborate spell in the passage above, for instance—we move through this reality with ease.