In "Three Against the Witch World" (1965), the triplets escape from witch-ruled Estcarp to the magical land of Escore. There, they accidentally destroy the false peace that had long abided between the great powers of Light and Dark. "Things awoke and gathered, and the land was troubled..." and the three learn that they must fight with the forces of Light, or be utterly annihilated by the Dark.
Unfortunately, in "Warlock of the Witch World" (1967), Kaththea is wooed by the fair-seeming Dinzil, who is actually a creature of the Great Dark Ones. She who was once a powerful witch is rescued by her brother, Kemoc, but because of her near-alliance with the Dark, she is stripped of her magic. No longer can she communicate mind-to-mind with her brothers, nor perform the simplest spell of healing.
A witch without her magic is a very dangerous thing to be in Escore: a vessel waiting to be filled by the Dark. In "Sorceress of the Witch World" (1968), Kaththea attempts to return over-mountain to Estcarp to seek help from the witches, but she is caught in an avalanche and then captured by a tribe of nomadic hunters.
Andre Norton is a scholar of Amerindian history and lore, and has incorporated her knowledge in many of her novels, e.g. "Sioux Spacemen" (1960), and "The Beast Master" (1959). The dog-sleds and temporary dwellings of the Vupsall, the people who capture Kaththea, are yet another example of Norton's borrowings from history and anthropology, although this particular Escorian tribe also works metal (maybe a touch of Finno-Ugric, rather than Amerindian).
At any rate, this author's careful attention to detail will bring to life the dimly-lit interior of Utta, the Wisewoman's tent where Kaththea begins to reacquire the magic that was her birthright.
'Sorceress' completes the very fine trilogy of the adventures of Kemoc, Kyllan, and Kaththea, the three children of Earth-born warrior, Simon Tregarth and his witch-wife, Jaelithe.