I'm not sure if it's exactly non binary but I LOVED Lythande by Marion Zimmer Bradley. The wizards in a particular order have to have a secret to keep their power, otherwise they lose their magic and can be killed by another blue star mage. It's kind of Highlander: 'there can only be one', except the focus is much more on Lythande as a person. She poses as a man as her secret and while she seems happy to have a female body, she also seems to have no problem being treated like a man. I believe her secret is so shocking because it is lore in the Blue Star mages that only men (rather biological males) can join this order, Lythande becomes one through a loophole or deception and then must retain the identity to keep her power.
I found this one interesting since there were many historical figures who were born female but cross dressed for their own reasons- many people claim the historical figures did so to be more free and have power, but I feel like they certainly had room for being transgender/bigender/something else along the gender spectrum. It had a nice feel to it and Lythande was really relatable for me. They are a strong willed and intelligent individual- mostly a person who fights for a good cause, but also a total action hero/heroine/wish there were a non gendered form of the word XD
I think it's nonbinary in the sense that Lythande is content with themselves as a person and I do really see them as someone who is pretty solidly non typical as far as personality and behavior compared to other characters who are pretty easily one particular gender or another. Just unique and a fun fantasy collection