Hah, great link. The two books I would recommend here are Simon Hornblower's The Greek World, 479-323 BC, and Terry Buckley's Aspects of Greek History 750-323 BC: A Source-based Approach.

Hornblower because he's fairly generous to Xenophon: he makes extensive use of him as a source, and doesn't say anything particularly negative about him. But he still points out that Diodorus is the more reliable source, especially in the wake of the 'Oxyrhynchus Historian' find. He's a good example of a historian using Xenophon as a source with caution.

Terry Buckley is far less generous. He allows that Xenophon's works can be enjoyed as memoirs, but states that as a historical source Xenophon is to be criticised for his 'blatant bias' and 'remarkable omissions'. With his Spartan sympathies, for example, Xenophon fails to record entire battles where the Spartans lost, while also exaggerating the significance of battles which the Spartans won. I think the segment is viewable via preview on Amazon, if you want to have a look yourself.

​

EDIT: Just to expand a little on the above, these are Buckley's reasons in more detail.

• Xenophon does not detail his methodology. Unlike Thucydides, Xenophon does not tell us how he gathered his sources or what his sources were.
• Xenophon is partisan. Xenophon believes the 'upper-class, pro-Spartan, pro-oligarchic supporters of Agesilaos' were 'concerned with the best interests of the Peloponnese' and gives their account of events. Thucydides can't be said to be nearly so favourable to the Athenians.
• Xenophon is selective in his reporting of battles. He 'dwells at excessive length on minor victories of the Spartans' (Buckley gives the example of the Tearless Battle against the Arcadians) and 'belittles major Theban successes.'
• Quoting Buckley: 'the Theban invasion of Laconia up to the very outskirts of the city of Sparta (previously believed to be impossible and therefore deeply humiliating for the Spartans) is portrayed as tentative, fearful and deliberately avoiding any clash with the Spartan hoplites.'
• Xenophon is overly didactic. He portrays Agesilaos as the ideal leader and as sharing the values that Xenophon cherishes: a strong military leader, pious, and antagonism to the Persians. He also tends to give stirring eulogies to people who enshrine the morals that Xenophon values, even if they ultimately make little contribution to events.

Broadly speaking, I think Xenophon has such a poor reputation because he positioned the Hellenica as a follow-up to Thucydides, and then did such a bad job of emulating Thucydides' careful scholarship. And, of course, it doesn't help that Diodorus' star has risen in the last century where Xenophon's has sunk a bit.