Where Thucydides breaks off, Xenophon takes up. Xenophon was a young man when the war ended, and a pupil of Socrates. After the war, he joined a Greek mercenary army taking part in Persia's civil war and is best known for writing the Anabasis, his account of how he got the army safely out after their side was defeated. But he wrote numerous other works, including the Hellenica, a history that takes up where Thucydides leaves off. Xenophon presupposes that his reader is familiar with Thucydides; he begins as abruptly as Thucydides ends, "After this, not many days later." Xenophon's history is considered rather "thin" compared to Thucydides, i.e., it tends to give less detail and fewer perspectives. But it describes the time of the Thirty Tyrants better than any other source we have. Xenophon was in Athens at the time of the Thirty and almost certainly witnessed their actions. He was also a pupil of Socrates and gives an account of the master including a description of what he was doing under the Thirty.
Xenophon was not the only historian of the period. Roman-era historian Diodorus Siculus says that not only Xenophon, but Theopompus, a historian whose work has been lost, as have the works of Cratippus, another contemporary historian. Diodorus, though writing centuries later, appears to have relied on these works as well as on Xenophon.
Plutarch does not have biographies of any of the principles in Athens at the time of the Thirty, but his biographies of Alcibiades and Lysander, the Spartan Admiral who finally defeated Athens and installed the Thirty, contain useful information about the city's downfall. Finally, under the earlier democracy, Athens, great center of culture and learning though it was, remained in many ways an oral culture. The speeches of the period were given orally and not written down. The restored democracy had professional speech writers who prepared speeches to be given to the Assembly and to juries. These speeches are all (intentionally) biased, but they are a useful source of information on Athens under the Thirty.
And, as before, modern historians Kagan and Grote have been useful in explaining and interpreting, though relying on incomplete information and a large measure of speculation. And there are minor sources as well.
So, on to the final years of the Peloponnesian War and the downfall of Athens.