The Two (Or Three) Ythrian Volumes

Poul Anderson's The People Of The Wind, a novel about the Terran War on Avalon, and The Earth Book Of Stormgate, a collection fictitiously compiled on Avalon, are companion volumes because:

the Earth Book collection includes the four short stories that are specifically about Ythrians and a fifth in which beings of that species play a major role;

in fact, more specifically, the collection begins with two stories about early human-Ythrian contact and ends with two stories about the joint human-Ythrian colonization of Avalon and, in the third last story, an Ythrian sees the shadow of God the Hunter across the Polesotechnic League;

the additional introductory passages in the Earth Book are not only based on The People Of The Wind but also provide a sequel to it;

thus, Hloch, who had served in space during the Terran War, subsequently visited Imperial planets as a crew member in a merchant ship, then returned to Avalon where he compiled the Earth Book at the request of the Wyvan Tariat and with cooperation from Arinnian/Christopher Holm who is a major character in The People Of The Wind while Tariat's parents, Lythran and Blawsa, also appear briefly.

The third and last volume with a role for an Ythrian is The Day Of Their Return. Ironically, the winged beings in the attached cover image are not Ythrians but Diomedeans because the Earth Book includes The Man Who Counts about Nicholas van Rijn's achievements on Diomedes. Hloch includes this narrative because:

van Rijn, an important historical figure, is central to it;

the events on Diomedes had important consequences for Hermes, the home planet of David Falkayn who was both van Rijn's protege and the Founder of the Avalonian colony;

flying intelligent beings are rare and of special interest to Ythrians;

Diomedes is a freak planet.