It is always a little tricky starting with the sources in a popular work of history, but O'Connell makes this interesting enough to hold non-historians, with personal background on Polybius and Livy and the other sources as well as an attempt to place them in the broader context of ancient military history. There are also some comparisons with modern wars and current American military concerns.
O'Connell proceeds to describe the protagonists. Roman military success rested not just on its army and tactical resources — there is a nice description of how a legion was made up and fought — but on a political system that (mostly) selected reasonable leaders and gave them the right incentives, and which was notable for its resilience in the face of failure. A potted history of Carthage, in contrast, covers their defeat in the First Punic War, the Mercenary Revolt, and the development of what was effectively a Barcid family fiefdom in south-eastern Spain.
There is some background on Hannibal and the Gauls before the description of the crossing of the Alps and the battles, notably on the Trebia and Lake Trasimene, which made it clear that Hannibal was a real threat and which temporarily induced the Romans to fall back on a "Fabian" delaying strategy. There follows the full account of Cannae itself, in which Hannibal exploited disagreements between the Roman commanders, used the heterogeneous components of his army to best effect in a subtle deployment on open ground, and enveloped and destroyed the opposing army. There is some fairly gruesome detail about how the day-long killing might have proceeded.
The aftermath of Cannae saw Hannibal's rejection of an immediate attack on Rome, instead commencing what was to be a decade-long attempt to build an anti-Roman alliance in southern Italy. This was to illustrate the resilience of Rome, which continued to field armies and learn from its mistakes, and the limitations of individual military genius. The Romans also fought on other fronts, notably in Sicily and Spain, which saw the rise of Publius Scipio, later "Africanus". Using "the ghosts of Cannae", the Roman soldiers who had survived that battle and been sent into exile as punishment for doing so, and an alliance with the Numidian Massinissa, Scipio built the army that would invade Africa, force Hannibal's return from Italy, and defeat him at Zama.
O'Connell touches briefly on subsequent history, on Hannibal's later career and on the Third Punic war and the destruction of Carthage, but also on longer-term implications, on the debate over the enduring damage done to southern Italy by Hannibal's decade-long residence there and on how Rome came to rely on charismatic generals. And an epilogue explains that, though the battle of Cannae has become one of the classic strategic exemplars, that only happened some time into the twentieth century.
The Ghosts of Cannae stays close to the evidence, but uses a fair bit of additional material and some speculation to help flesh out the sketchy and sometimes unsatisfying narrative that that offers us. In the account of Cannae, for example, there are digressions into the psychology of battle and the likelihood of soldiers who find themselves in an adverse situation and unable to take action going into a kind of fugue state. And there is speculation about such matters as the role of the legiones Cannenses — the "ghosts" of the title — in events in Sicily.
O'Connell provides enough detail for the reader to see through them, but his summaries sometimes seem to fall back on stereotypes: "the Carthaginians were good at business and bad at war", for example. And he perhaps neglects the ideological elements of the war: he is dismissive of the Carthaginian use of war elephants, for example, evaluating them purely on their effectiveness in battle. (There is a contrast here with Richard Miles' treatment in Carthage Must be Destroyed, which emphasizes the role of religious rituals and Hannibal's identification with Heracles.)
The Ghosts of Cannae contained much that was new to me and is a nicely put together synthesis of the ancient sources and secondary scholarship. Anyone interested in the military history of the classical world should find it engrossing.