Eon is hiding a secret: she is actually Eona. In a fantasy mileau heavily influenced by feudal China, 12 spirit dragons (one for each of the years of the zodiac) help rule the land with the humans they are bonded to. They all serve under the Emperor. Being a Dragoneye wears out the body, so after a 12 year training period and a 12 year reign, the retired Dragoneye has the stamina of a old man, and a scant few years of retirement.
Because of the girl-disguised as boy motif, and the setting amidst the military training barracks, the obvious comparison is Tamara Pierce's Alanna's books, but Eon: Dragoneye Reborn take a much grittier tone. Eona is not only hiding her gender, she is also coping with a painful lame leg. As an awkward repressed memory resurfaces, Eon realizes that that her master was actually the one to inflict the injury, gambling on the fact that her deformity will allow her to avoid the locker room, all the better to conceal her secret. Of course, as a woman disguised as a boy, one of her prime concerns is how to cover her menstruation. This is discussed in frank detail and the author doesn't take an easy out of providing some magical herb which ceases her cycles, either.
The author plays with every possible variation of gender and class roles, including several different types of eunuchs, men and ladies of the court, serving maids, and contraires (biologically male, but they dress and live as women.)
At the tournament where candidates battle each other for the opportunity to become the next Dragoneye, Eona is nearly chosen by the Rat Dragon, but at the last moment she is instead chosen by the Mirror Dragon, missing for these past 500 years. Believing her to be a fellow eunuch, Ryoko and his mistress Lady Dela (a contraire) take Eona under their wing when Eona's victory elevates her to the prickly intrigues of the imperial court.
Eona spends most of her time trying to subdue her female or "Moon" energy, even going so far as to take steroids. It's only at the very end that Eona realizes the Mirror Dragon is actually female as well, and embraces her femininity to fully access her power.
Long battle scenes, and a plot that moved a bit slowly in places can be forgiven for the richly detailed world, complete with several nations, competing social classes, and a complex backstory. Goodman's exploration of gender settles in nicely somewhere between Tamara Pierce's Alanna series and Ursula LeGuin's Left Hand of Darkness. The highly anticipated sequel, Eona: The Last Dragoneye isn't due to be published until 2010, but we can only hope that Eona will be marshaling her magical gifts and martial training to meet the challenges that will surely be facing her ahead.