Books like Heris Serrano

Fleet officer Heris Serrano came from a family of Fleet officers, so when a lying superior forced her to resign, life lost all meaning. To pay the bills, she became Captain of a rich old lady's interstellar luxury yacht, adding insult to injury. But Cecelia, the rich old lady, had more brains than most admirals Heris had known, and before it was all over, Heris would have a chance to rejoin her beloved space navy -- if she could manage to stop an invading armada.