With the Lightnings by David Drake

Basically space opera - but good fun. A young lieutenant saves the world with very little but his charm and good luck - and the able assistance of a highly skilled librarian.

In all fairness, I suppose Adele is a bit more than just a librarian. Her passion is information and the gathering thereof - we might call her a hacker. Not to mention the fact that she is a dead shot with a pistol and not seriously squeamish about using one.

Adele and Daniel's first meeting is not auspicious. After all, Daniel's father is the biggest dog on their home planet of Cinnabar and the head of the tribunal that sentenced Adele's entire family to death - not excepting her ten-year-old sister, Agatha. Adele survived by being off-planet at the time. However, events conspire to make them partners in thwarting a coup which is intended to be a first step to bringing down the Republic.

It may be noted that Daniel is only available to save the Republic because of his complete insignificance in the scheme of things in the RCN - Republic of Cinnabar Navy.