This is a collection of stories from various points in the history of Havnor and the surrounding archipelago in the time before Ged. It starts at a time when witches are being actively hunted down, and the tale of one such wizard and his struggle to overcome an appalling, oppressive, and evil regime in his attempt to free himself and others like him and rid the world of the evil. Earthsea is a very different place at this point, and it's a fascinating insight that's far more interesting than certain sci-fi "prequels" or other elaborations.
Similar stories follow, including the actual story of the earthquake arrested by two magicians hinted at in earlier books—but got quite wrong.
One of the stories tells of the founding of Roke, the island of wizards. It's a story of how women lost their footing as equals to men in the wizardry game.
All of these tales serve three things: to provide le Guin's usual engaging tales in their own right; to understand feminism through a neutral context; and to help us understand her vision of magic. That vision, I think, is really quite brilliant. It's amazing how she manages to convey so much, and it shows how a lot of fantasy makes magic seem like a child's cartoon.