The Heretic is the latest in the long-running General series by David Drake and one of several writers.  (The original series was co-written with S.M. Stirling. The first book in the latter series was co-written with Stirling, but Drake has switched co-writers since them.) I do not recommend the original series unless you are completely acclimated to Stirling’s early work, which was heavy on the “I have to show you how grotty and violent the world is with graphic eroticized sexual violence.”) 
 

The basic synopsis of the series runs something like this: Once upon a time there was a vast galactic civilization that collapsed, and that collapse slid downward into utter-yet-strangely-copacetic-with-World-History barbarism.