No Man of Woman Born is just... super duper good, OMG. I recommend GumRoad for the ebook and/or audiobook, but the other big sites have it as well, and there is a paperback version available. The second book in xer Earthside series, Survival Route (which can be read on its own), also has a nonbinary main character, though not a POV character: A half-Japanese demigirl who uses the same pronouns as Mardoll.
While I'm at it, here's a rec list from Shira Glassman (a Jewish bisexual cis woman who writes adorable queer fiction; no nonbinary MCs yet, but I suspect it is only a matter of time) of SFF stories featuring nonbinary characters, which itself links to a further rec list for SFF stories featuring nonbinary characters.
no subject
While I'm at it, here's a rec list from Shira Glassman (a Jewish bisexual cis woman who writes adorable queer fiction; no nonbinary MCs yet, but I suspect it is only a matter of time) of SFF stories featuring nonbinary characters, which itself links to a further rec list for SFF stories featuring nonbinary characters.