I recently finished a novel simply titled Caesar - on Gaius Julius Caesar's campaigns in Gaul and his political then physical war against the Senate. Turns out it is the fifth book in Colleen McCullough's Masters of Rome series, of which I've now read the first and am well through the second. Anyone who is interested in a deep, detailed view of how the republic functioned need look no further. http://en.wikipedia.org/wiki/Masters_of_Rome
Sometimes the novels read like history books, and in some cases the narrative's pacing is slow as it deals with secondary characters, but it serves to bring unparalleled empathy for the lives of even the most famous and powerful figures. The battle scenes aren't as gritty and visceral as true war novels - mostly battles are seen via a general's eyes atop a nearby hill - but they serve well enough to grasp how the battles played out on a macro level and what ramifications their conduct would have on the notable figures.
Be prepared to learn more about the republic's customs, culture, and laws than you bargained for and to get lost in an expertly manifested ancient Mediterranean world.