Its a bit dated but Wallbank The Hellenistic World (1981) is excellent on the Diadochi and their model of governance and interactions.
Polybius in the Penguin Classics series The Rise of the Roman Republic - the chosen selection mostly focuses on the Punic Wars and the Second Macedonian war. Its far more readable than Livy over the same material.
Again Penguin Classics, Cicero Selected Works, as good on the internal dynamics of the late Republic as anything i've read.
Catullus poems, again Penguin Classics, utterly filthy, just the thing to get your average 16 year old interested in learning Latin, also surprisingly good insights into the late Republic/early Empire.
Robert Graves I, Claudius, Claudius the God, ok fiction but still a good read and covers the period up to 68AD very well.
Tacitus - various but his life of Agricola is a really interesting study of Roman Britain from 70-90AD, and the Histories covers the civil war of 67-70AD.
Mary Beard's SPQR is a good general history of Rome.