Digby, of course, picked some good points. One sometimes wonders if publishers read books; at times, they seem to make choices that no book-reader would ever contemplate!
Some additional points come to mind:
-- a clear articulation of relationship between events--even when the relationship is uncertain. Often information or chronology is fudged when information is skimpy, rather than a clear acknowledgement made of "history's" uncertainty. (I appreciated this approach in Rory Muir's "Salamanca"--even though he is numbered among the living!) It seems all too easy, at times, to let the quest for a fully-fleshed narrative overwhelm the historical absence of adequate fact for aspects of events. That "fudgery" can play later havoc with real comprehension for the reader. :o)
-- inclusion of particularly interesting supporting documentation. Of course the vast percentage of documentation to support the story cannot be included, but Chambers' "Busaco" is an outstanding example of a tidy battle-study that provided a proportionately substantial appendix full of reports and letters that mere mortals would have had a hard time locating. The participants' own words lent rich texture to the framework the author had hammered out.
-- a good-faith inter-relationship between events and the people who participated in, caused, or suffered from, them. Whether a subscriber to the "great man" theory or a proponent of a tide of events in human affairs, "his" story and "her" story is ultimately about people, of course, how events affect people and people, events. Doesn't require a biography of each person involved, but at least a sufficient thumbnail of an adequate number and range of participants that one catches at the human content of history. It helps if one can care a bit about the participants, have a sense of why they made their choices...or didn't. Peter Cozzens does this very well in his several American Civil War campaign/battle studies. In fact, he provides excellent, well-placed, and thoroughly integrated maps, too. He is also very adept at dissecting action in a way that allows one to appreciate the parts, and still arrive at an sense of the whole.
Those would by my contributions to the mystery of history!
Cheers - Howie