Angolin by C. E. Taylor

A hidden people not so hidden anymore.
The Hidden Realm of Angolin has existed in peaceful isolation for centuries, but that's about to change. The enemy across the Abyss has discovered them and is intent on conquest. Lieutenant Dharmen Tate has a great love for his homeland and a greater understanding of its position in the world. When he discovers a plot by fellow officers to join the enemy and overthrow Angolin, he and his colleagues spring into action to defend the Hidden Realm from its foes-those without, and those within. The ensuing struggle magnifies Angolin's vulnerability and exposes the collusion of a growing body of its citizens, signifying an uncertain and bloody future.
For readers who enjoy Star Wars: Bloodline by Claudia Gray, Embassytown by China Mieville, and Catalyst of Sorrows by Margaret Wander Bonnano.