What the book is
The Cyropaedia is a long work — eight books — by an Athenian soldier-historian about a Persian king who had lived almost two centuries before. Xenophon was not writing history in any strict sense. The historical Cyrus is recoverable from Herodotus, from the Cyrus Cylinder and the other Near Eastern inscriptions, and from the Babylonian sources; Xenophon's Cyrus departs from this material at almost every turn that matters. What he was writing was something for which there was no Greek precedent and few subsequent imitators: a long, dramatic, partly-dialogue work whose principal subject is the formation of a ruler under conditions in which his character will decide the fate of an empire.
What it asks
The question the Cyropaedia presses is not "who was Cyrus" but "what is a ruler, and how does he come to be one." The answer the book gives is structural: Cyrus is shaped first by a specific upbringing (the Persian education described in Book I, an idealised version of what Xenophon believed the historical Persians to have done), then by a set of teachers and elders, then by experience in command, and most of all by the practice of governing relationships — with subordinates, with allies, with subject peoples, with rivals. The book is full of dialogues; the Cyrus who emerges is the Cyrus who has worked through them.
Why it is not a handbook
The temptation, especially in the early-modern reception, was to treat the Cyropaedia as a mirror-for-princes handbook — a manual extractable into rules. Machiavelli's Prince explicitly cites it and reads it partly that way. The book itself resists the treatment. The Cyrus who emerges from the long sequence of incidents is not the Cyrus of any one episode. The lesson, to the extent there is one, is that good rulership is the accumulated effect of a particular kind of character formed across a particular kind of life. There is no shortcut.
Xenophon also writes a closing book (VIII) on the rapid decay of Cyrus' settlement after his death. This is not an accident. The book is not telling the reader that Cyrus' methods produced a regime that survived; it is telling the reader that even a ruler of exceptional character could not transmit his rule to successors who had not been formed in the same way. The implication is sober.
Why the European tradition kept reading it
The European inheritance of the Cyropaedia is unexpectedly large. Cicero refers to it. Scipio Africanus is said by Cicero to have kept it always at hand. The early-modern republican and humanist reception — Erasmus, Sidney's Defence of Poesy — read it carefully. Montesquieu and the American founders received it. The reason is the same reason the Greek tradition kept reading Plato's Republic — not because they thought the regime described could be implemented, but because the inquiry into how a particular kind of person is formed is not exhausted by the particular text in which it first appears.
Why the platform reads it
The platform reads the Cyropaedia for the question, not for the recipe. The question — what is the education that produces a ruler who governs well, and what conditions make such an education possible — is one a republic faces as urgently as a monarchy. The American founders, whose explicit project was a republic, read the Cyropaedia because they understood that even a republic needs citizens fit to lead it, and the question of how such citizens are formed is not bounded by the regime that needs them.