A Talent For War by Jack McDevitt (currently 99p on kindle and kobo)

A Talent for War is a science fiction mystery novel by American writer Jack McDevitt,[1] the story of a search by Alex Benedict, the protagonist, to discover the nature of a mysterious project Alex's uncle had been working on when the interstellar passenger ship, on which his uncle was a passenger, was lost in space. This investigation leads deep into the history of a war between human civilization and a neighboring alien civilization and challenges the foundation mythos of the current human government.

The first three books in the series are currently 99p on kindle and kobo.