A good biography should do exactly what you want. For Augustus I would recommend The Roman Revolution by Ronald Syme, The Legacy of Caesar by Osgood, and Rome's Revolution by Richard Alston. These books aren't very interested in Augustus's reign but more in how Octavian became Augustus. Syme actually begins with Pompey and Caesar but the other two begin with the death of Caesar and focus on the Second Triumvirate. Hopefully those will interest you. As for the city of marble stuff you might be interested in Galinsky's Augustan Culture which talks about the impact Augustus had on construction and the literary arts and how he used cultural motifs to communicate with his subjects and how his subjects used those motifs to communicate with him. Before Galinsky, Zanker covered the same topics in his The Power of Images in the Age of Augustus. Galinsky is a reassessment of Zanker.

I also recommend you read the ancient biographies of Augustus and the Julio-Claudians in Suetonius's The Twelve Caesars. They are quite entertaining and offer a second-century Roman's take on the emperors. Tacitus (Suetonius's contemporary) offers his own thoughts on the matter in Book 1 of his Annals, which starts at the death of Augustus but focuses on the impacts monarchy had on Rome and the way Romans responded to it. Another major source on Augustus is Cassius Dio but he writes in the third century in Greek so his evaluation is less poignant than the other two.

Often it is hard for people to understand when historians or biographers are "evaluating" their subject as opposed to building a narrative based on facts. In reality, ancient history is almost all evaluation and any narrative is based on the judgements of the historian on what is a trustworthy fact and what is not. That's why I have suggested multiple books on the same topic. One biographer may paint Augustus as the consummate strategist, playing 4D chess over the bodies of Cicero and Antony. Another, using the exact same sources as the former, may consider Augustus the lucky lone survivor of a generation of aristocratic hemorrhaging. One might compare Augustus to Alexander, another to Hitler. Was Augustus playing against the rules of the Roman Republic or was he so good at the Roman Republic that he won the game by such a huge margin other people stopped playing? These are all different takes on Augustus, found in biographies of him and his successors, that are all made using essentially the same details but evaluated differently. That's why I'm recommending you read a multiple authors' takes on the same events. I highly recommend you start with ancient authors, though.

Gibbon does not cover Augustus that much, he starts at around 98 AD (hence why it's named Decline and Fall not Rise and Fall). For a good book on Augustus, and many of the other early emperors, A Dark History: The Roman Emperors is very good, it tells the story of how they got into power, their time in the position of emperor, and their eventual demises.