Retcon And Overlapping Narratives

The Young Flandry Trilogy and its two sequels, "Outpost of Empire" and The Day Of Their Return, are one long retcon (retroactive continuity) because Dominic Flandry and the Terran Empire were introduced in the first Captain Flandry story published years earlier. The Day Of Their Return also completes the story of human-Ythrian interaction because it:

features a new Ythrian character, Erannath of Stormgate Choth on Avalon;

informs us that human governmental institutions have died out on Avalon;

also informs us that the Domain of Ythri now cooperates with the Terran Empire against the Merseian Roidhunate.

Apart from one later reference by Flandry, this information in The Day Of Their Return is the last that we read about the biracial colony on Avalon. Narratives overlap. The Technic History comprises League and Empire linked by Ythrians with a few other works set both before and after. It is good that one cover illustration for The Day Of Their Return shows an Ythrian in flight.