I think this is the first Witch World novel I read. It may even be the first Andre Norton book that I ever read. And even though it's the third Witch World novel, Witch World is like Discworld; a bunch of stories that happen to have the same setting. Within the setting there are sequences it's best to read in order, but Year is the beginning of such a sequence.
It opens with Gillan trapped in the Abbey where many women had taken refuge after the Alizon attacks. Except that she was found, prisoner, on one of their ships, and she is obviously not one of the race that live in High Hallack. The lord who sent her there, and his lady, are both dead, and while many young women are going to their weddings, she has nowhere to go and no certain status even in the Abbey.
Meanwhile, the attacks are over, and the lords are readying to pay the price. The mysterious Were Riders fought at their side and agreed to leave the waste so they could take it over. In return, the Riders wanted brides to take with them, twelve and one maidens of gentle blood. Twelve of the brides are being brought to the Abbey. Gillan thinks she knows who the one will be -- but they are all surprised by who is it. And the woman herself is hysterical, and, as another observes, will not survive. With some deft herb work, Gillan inserts herself. Anything is better than being trapped.
When in due course they arrive at the appointed location and are left there, the next morning they go up a slope to find a grassy sward where it is inexplicably spring, with cloaks spread out over it. Or so the other brides see. When Gillan looks, she sees it, and then it fades to winter, with rather different looking cloaks on it. When each of the other brides has taken up the cloaks, she eventually takes one, and finds herself thus chosen by Herrel, whom, it turns out, is considered by the Riders the least of them.
This has consequences. It involves dreadful dreams, a spell on Gillan's horse, a magical gate, a challenge enabled by moly, and some (not many) discoveries by Gillan about her unremembered past and why she can see through the magic.