Jaran by Kate Elliott (The Jaran#1) 1992 (494 pgs)
Tess is full of doubts and intent on evading the heavy expectations that result from her position as sister and heir to the only human duke in an alien empire. When she heads for a vacation on a backwater planet in her brother’s domain, she instead stumbles upon a group of aliens violating territorial agreements by setting out on an expedition across the forbidden zone. On instinct she follows them, determined to aid her brother and his planned human rebellion.
She finds herself alone among the Jaran, a warlike equestrian nomadic society that rules the plains. There she works to earn the acceptance and respect of the tribe while trying to discover the aliens’ true purpose.
Great fantasy series I like to re-read every once in a while. The world is richly developed with fascinating characters and plot.