The Steerswoman series (Rosemary Kirstein)

This is a science fantasy series that takes culture seriously. Even the premise is focused on Steerswomen as custodians of knowledge. They must answer any question put to them to the best of their knowledge, but they may also ask any question and expect to get the truth. This makes it very, very dangerous for one Steerswoman to start asking questions that powerful people don't want her to answer.

One caveat: the series is still unfinished. If you know how much I hate reading unfinished series, you'll have a good sense of just how great these books are.