The Killer Angels is set in the American Civil War, so not really as far back as you described,but it really set the standard for modern historical fiction. The author used primary sources (letters, journals etc of the officers on both sides) to get as accurate a depiction of events, personalities, and inner thoughts as possible. It is the book that the movie Gettysburg is based on, but written like 20-30 years before the movie.

You did say you were open to worthy suggestions and I think this is really a worthy one.