Colleen McCullough's Masters of Rome series was pretty good.

While not focusing solely on the military aspects of the Late Republic (although she does go into some great deal about the Marian reforms), she does an excellent job in my opinion of detailing the campaigns of the renegade Quintus Sertorius as well as the pontic wars between King Mithridates and the Republic. The Social and Civil wars and the guerilla campaigns of the Samnites and Marsian tribes are also well done I think.

The First Man in Rome
Fortune's Favourites
Caesar's Women
Caesar
The October Horse

Good historical fiction that aims for realism as opposed to strict historical accuracy. When it comes to the minutae of wearing a toga or how to weaken the shaft of a pila though the author makes sure to get it right.