Currency of War. Imperials Saga, #4

It's been two hundred years since the Solar League has faced an alien threat, but now one is on their doorstep, and it has fallen to the Empress Mercedes de Arango to inspire and lead her people even as the aliens rain down death on League worlds. The need for competent military officers is desperate, so Mercedes reaches out to her secret love and begs Tracy to return to active duty - even though that might risk Tracy learning the truth about the crown prince of the Solar League.
In the midst of this war against outside invaders there is also a looming threat of civil war with both Mercedes' youngest sister and her son being pawns in this game of thrones. Once again it is Tracy who rescues both the sister and the son only to discover a devasting secret that will threaten the stability of the throne.