here) comprises no less than eight works of dissimilar types:
five short stories;
one set of introductions and a conclusion specially written for an omnibus collection, The Earth Book Of Stormgate -
- and this is not really a series. Instead, the Ythrians weave in and out of Anderson's major future history series, the History of Technic Civilization. The omnibus collection includes the five short stories so we are talking about only three volumes here although, of course, Baen Books have subsequently collected the entire History into the seven omnibus volumes of The Technic Civilization Saga, thankfully preserving the Earth Book introductions and conclusion so that Saga, Volumes I-III, read like an expanded Earth Book.
Only the first of the short stories is set on Ythri;
three stories and one novel are set on the joint human-Ythrian colony planet, Avalon;
the first of these three stories describes not the colonization but the earlier exploration of the planet that came to be called "Avalon";
the second of the novels, set much later, features an Ythrian on Aeneas but thus also discloses significant cooperation between the Domain of Ythri and the Terran Empire which had been at war in the first of the two novels;
the remaining short story features an Ythrian spaceship, thus showing how far the Ythrians had progressed since first contact in the very first story.