The Steerswoman series by Rosemary Kirstein (1989–2004)

I found out about these because Randall Munroe has referenced it in xkcd and also mentioned once that they were up his alley and he’d been recommending them. It’s really best to go into them without knowing a whole lot, but the basic premise is that the main character in this world is a Steerswoman, part of an order of traveling gatherers and sharers of knowledge who must always answer all questions as truthfully as they can, but if a Steerswoman asks you a question and you either refuse to answer or don’t answer truthfully, no Steerswoman will ever answer your questions again. At first it seems like fairly normal fantasy world-building—there are wizards here—but it ends up going in some unexpected directions.

Caveat: There are four books so far, and supposedly it’s planned to be seven altogether, but the fourth one came out in 2004 and there’s been nothing since then, so...yeah, we're past even George R.R. Martin territory at this point. As of 2018 she had said she was working on books five and six, so I’m still holding out hope. But anyway, I enjoyed the first four enough that I’ll still be glad I read them even if the series never fully resolves.