I agree with Kyle Murao about Rubicon, which is the best book to give to a newcomer to Roman history. I would also suggest the History of Rome by Theodor Mommsen: although it is 150 years old (and thus public domain), Mommsen was a superb scholar with a thorough understanding of all the ancient sources. read more
The Roman Republic (Latin: Res publica Romana; Classical Latin: [ˈreːs ˈpuːb.lɪ.ka roːˈmaː.na]) was the era of classical Roman civilization beginning with the overthrow of the Roman Kingdom, traditionally dated to 509 BC, and ending in 27 BC with the establishment of the Roman Empire. read more