Actually, it is not nine books but nine chapters of a great book, each of them dedicated to one of the nine muses, "so the deeds of men do not fade in time".
Book I is dedicated to Clio, muse of History. It tells it was the kidnapping of Io, Princess of Argos, by the Phoenicians, the cause of discord between Greeks and Asia. And Croesus, of Lydian lineage, was the first among the barbarians who subjugated Greek peoples. Croesus subdued the Greeks of Asia and turned Sardis into a thriving city where wise men as Solon came to. The Lydian faced the Persians ruled by Cyrus who beat him and took Lydia and its capital.
After this description, Herodotus deals with telling us who Cyrus was. He goes back to the Assyrians who perished at the hands of the Medes, up to telling us the legendary origins of the Persian king, son of Cambyses.
The author intersperses descriptions of the Persian customs. And then jumps to list Greek cities and leagues in Asia. Then he relates the devastation of Asian Ionia by a general of Cyrus, Harpagus. Finally, he describes customs of the Assyrians, mainly in Babylonia, and culminates with the death of Cyrus at the hands of Massagetae and their queen, Tomiris.
Book II is devoted to Euterpe, muse of Music. This is the book of the acts of Cambyses, and Egypt: its religion, kinship with the Greeks, the art of divination, oracles, public holidays, rituals and ceremonies and sacred animals, and their dynasties. Herodotus states that most of the Greek gods came from Egypt, at least their names.
Moreover, he inserts a story about Paris and Helen explaining why the Trojans did not return her to Sparta. We also learn that Herodotus wrote his great book during the time of the Persian dominion in Egypt.
Continuing History, Book III is dedicated to Thalia, muse of Comedy and light poetry. It follows the life and death of Cambyses, son of Cyrus, in Egypt. It also tells about Samos wars against the Lacedaemonian and the taking of Babylon. Likewise, there is a description of the boundaries of the known lands, which, for the period, were India, Arabia and Ethiopia. He also mentions Darius'' plot to ascend the throne.
The IV is the Book of Melpomene, the muse of Tragedy. Much of it is dedicated to the fearsome Scythians and their wars against Darius and how the Persian laid a pontoon bridge with boats over the Bosporus to conquer them with 70,000 soldiers.
As he proceeded with Egypt, Herodotus describes all sorts of customs and sacrifices, as well as those of Libya (that is to say, the rest of Africa, besides Egypt and Ethiopia)
Continuing the story of the wars against the Persian, Book V, Terpsichore''s (muse of Dance), mentions the campaign of Megabazo, Darius''s general, against Thrace. Herodotus takes the opportunity to mention the introduction of writing in Greece by the Phoenicians and the wars of the Athenians, especially against Aegina which was always a threat to the inhabitants of Athens.
However, the persistent intimidation of the Persians caused the enmity between the Greeks did not last. Thus Book VI central theme, dedicated to Erato, muse of choral lyric, is the battle of Marathon. The Athenian general Miltiades stands out though he will have then a nefarious ending. It is also mentioned the Persian campaign against Miletus.
Once aware of the defeat at Marathon, Darius got ready to go again against the Greeks, but he died all of a sudden. This and what happened afterwards with his successor Xerxes, the battle of Thermopylae and Spartan Leonidas and his 300''s courageous actions is what Book VII is about, dedicated to Polyhymnia, muse of Mime. It is also about the detailed description of Xerxes'' vast army, as well as the acts of Themistocles, an Athenian general.
To develop Book VIII (Urania, muse of Astronomy), Herodotus focuses on Salamis awesome battle and Themistocles'' ruses to carry it out. He relates Xerxes'' withdrawal in defeat to Asia and the ravages it caused in its wake.
However, Xerxes left his general Mardonio in Thessaly with 30,000 men. This revolted against Athens, devastated and burnt it down. But in the battle of Plataea, the Greeks, with Pausanias, put an end to Persian domination in Greece. So with the facts of Mardonius, his death and Pausanias'' victory, as well as the withdrawal of Xerxes'' last general, Artabazus, to Asia, Book IX, devoted to Calliope, the muse of epic poetry, closes.
The great appeal of Herodotus and his Books stems from the fact that he does not just tell about the arisen battles between Greeks and Persians in order to dominate the Peloponnese. Moving between reality, myth and legend, Halicarnassus protohistorian uses it up to teach us lessons of ethnography, folklore, anthropology, religion and even geography and engineering. All this dotted with attractive anecdotes and wise sentences.