The Worth of a Shell - The Stone Moon Trilogy, #1

Born to a harsh world, we Jokka have evolved three sexes to survive: neuter, male and female. Twice in our lives we may change from one to another. A change we accept with grace... or resignation. It was our way. ...until one female defied all tradition: Dlane Ashoi-anadi, revolutionary, intentionally childless, runaway. This is not her story. This is mine. I am Thenet Reña-eperu, female-guardian, voice of orthodoxy...
and Dlane's first and dearest companion. This is the tale of how we changed each other... and how that changed everything. Book 1 of the Stone Moon Trilogy (continues with Book 2: Pearl in the Void, and Book 3: A Bloom in the North).