The conclusion of the Ozark Trilogy is a wee bit daft, like the rest of the series, but it begins on a seriously dark note. The Travellers, the dour, black-clad "fundamentalists" of the Planet Ozark, having now successfully isolated themselves from the rest of the world, have gone seriously Hillbilly-Taliban, and the book opens with them whipping a ten-year-old girl to death.
In the last book, Ozark's Magicians of Rank put Responsible of Brightwater in a magical coma. It turns out that Responsible was the one holding the planet together, in more ways than one. Without her, not only does the Confederation of Continents fall apart, causing the entire planet to descend into anarchy, but magic also goes away. The Magicians of Rank no longer have any power, and without magic and technology, starvation, disease, and war follows.
Troublesome, who everyone insists on calling wicked and evil and without human feeling, goes on a quest to find out what happened to her sister and then how to make it right. Meanwhile, the Garnet Ring, the villains of the trilogy even though they spend all three books entirely off-stage, have positioned giant magical diamonds over all twelve Castles, portending doom for everyone.
Troublesome, of course, revives Responsible, who saves the day by televising a morale-boosting call to action to the rest of the planet.
All the charming, daft worldbuilding details remain charming, daft, and never really explained. Telepathic sentient mules, two races of aliens living on the same planet but hardly ever seen, an imperialistic magical interstellar empire that wants to take over Ozark but won't because of some convenient self-imposed rules, and the fate of the Planet Ozark resting in the hands of fifteen-year-old girls.
This is a fun trilogy, not exactly tight in terms of consistency or plausibility, but it's offbeat and there are some imaginative gems and a few clever ideas about language and society. The series shares a lot in common with a unicorns-and-dragons fantasy series, reminiscent of Anne McCaffrey or Mercedes Lackey or Piers Anthony, but very mildly feminist, and with certain expectations subverted.