Building On A Base

A future history series is not linear but pyramidal. The earliest installments are the base:

in Heinlein's Future History, stories of technological advances;
in Anderson's Psychotechnic History, stories of recovery from nuclear war;
in Anderson's Technic History, stories introducing Ythrians and the League;
in Niven's Known Space future history, stories of interplanetary exploration.

In each fictitious history, the concluding story is the peak of the pyramid. Intermediate stories build on the base by referring back to already established aspects of the history. Lazarus Long refers to the Venerians, the Prophets and Coventry and makes dramatic use of "The Green Hills of Earth."

In Anderson's The Day Of Their Return, Ivar Frederiksen joins a band of the tinerans, or tinerants, who had appeared briefly in The Rebel Worlds and:

"A shape blotted out the sun. They bounded to their feet."
-Poul Anderson, Captain Flandry: Defender Of The Terran Empire (New York, 2010), p. 136.

"They" are Ivar and two tinerans. The "shape" is an Ythrian landing! See image. Erannath of Stormgate choth on Avalon introduces himself and the history of Terran-Ythrian relationships is summarized. When Erannath reminisces about Avalon so can regular readers. We remember The Earth Book Of Stormgate which, according to its uncharacteristically accurate cover blurb, "...spans, illuminates and completes..." the history of the Polesotechnic League. Erannath's arrival is the best "building on a base" that we could have asked for.