“The Steerswoman” by Rosemary Kirstein is great. It’s the first book in an ongoing series that is great.
The series follows Rowan, a “steerwoman”, in a vaguely medieval setting. Steerswomen wander the world, making maps, collecting knowledge, and figuring things out, until they get too old to travel–at which point they go back to the Archives and help organize and train.
A Steerswoman must answer honestly any question asked of her. However, anyone who lies or refuses to answer a question that a Steerswoman asks is placed under the “Steerswoman ban”, which means no Steerswoman will ever answer your questions again.
The Steerswomen are well-liked, and everyone considers them helpful and good, except for the wizards. All the wizards are under the ban, because they refuse to discuss how their magic works.
At the beginning of this novel, Rowan decides to investigate these flat purple and silver jewels that have started to show up. Before long, the wizards are hunting her down. WHY?! WHY?! YOU’LL HAVE TO READ IT TO FIND OUT.