They’re not too good, but the most prolific writers of historical military fiction are leftists and Moslems.
Someone else here will have to help you out, I really don’t read fiction anymore.
The quasi-historical fiction I read in high school included “Fortune Made His Sword” (Henry V; I don’t know who wrote it), the sort of serial of novels that included “Eagle of the Ninth” (don’t know who wrote those either), and the Mary Stewart books on the Arthurian legends.
Eagle of the Ninth is being made into a movie. Coming out in September I believe.