Physician Anna Zarides needs to find out who really murdered Besarion Comnenos, a Byzantine nobleman, because her twin brother Justinian Lascaris has been exiled and imprisoned in a Syrian monastery for his supposed part in that murder. In order to move freely through Constantinople, and to treat any patient who may need her help, Anna disguises herself as a eunuch and takes the name Anatastius Zarides. With two servants from her family's small home town, she moves to Constantinople and begins building a medical practice that soon includes such notables as Zoe Chrysaphes, mother-in-law of Besarion Comnenos and former lover of Emperor Michael Palaeologus, and Orthodox Bishop Constantine. She slowly learns much about the intrigues of church and state (which are really the same institution) in Byzantium as her half of Christian civilization struggles with the dilemma of forced reunification with the Latin church. For it's either accept reunification, on the Pope's terms, or know for sure that the invasion people like Zoe Chrysaphes remember with horror (from 70 years ago) will happen again.
Anna's growing friendship with a half-Venetian, half-Byzantine sailor who is pursuing his own quest for personal truth proves to be the story's wild card. Much of the tension comes from Anna's rightly fearing discovery - for if she's found out to be a woman masquerading as a eunuch, the penalty will be severe and her ability to help Justinian will be gone forever. The rest comes from the politics of the era, which I found difficult to follow at times because the cast of characters was so vast. The position that eunuchs occupied in Byzantine culture intrigued me, just as it clearly intrigued the author. This "third gender" gives the book its title, and it also proves a major motive for highly placed Byzantines - who, like Bishop Constantine, most certainly could be eunuchs - to refuse reunification with the Latin church, to whom eunuchs are at best a puzzle and at worst an abomination.
A very good read, although Perry's hand isn't as sure with this universe and these characters as it is in her Victorian mystery series.
--Reviewed by Nina M. Osier, author of 2005 science fiction EPPIE winner "Regs"