I'd certainly never recommend Holland, at least not Rubicon...I can think of few books I'd recommend on the subject less...And Goldsworthy, excellent as he is for military matters, is quite often out of his depth when he ventures into politics. Have you considered looking at our Book List? We classicists don't often update our sections, certainly not as much as some of the other specialists, and perusing the "Ancient Rome" section I can see a number of things I'd add or modify but am too lazy to do so. Nevertheless, it's a perfectly adequate place to start, without any more mucking about in whatever Holland feels like saying. Besides that you really ought to read the classical texts themselves