The books are Cicero by Anthony Everitt which is a great book but still pop history and then Rubicon by Tom Holland which is... well, you probably already have your own opinion on it.
Any recommendations?
The Roman Revolution by Ronald Syme is a very academic book that focuses on the fall of the Roman Republic and the rise of the Julio-Claudian dynasty. Syme gets some criticism for being biased in Augustus's favor, but even the critics acknowledge that his work is accurate and well-researched. I read Cicero and Augustus by Anthony Everitt before I read The Roman Revolution, and they were a good intro to the subject. The Roman Revolution probably would've been too difficult for me to follow without them.