Hloch And Sandra Miesel

Five future historical volumes by Poul Anderson are enriched by interstitial material:

The Psychotechnic History
The Psychotechnic League
Cold Victory
Starship

The Technic History
The Earth Book Of Stormgate
The Long Night

Sandra Miesel's insterstitial passages pull together the Psychotechnic History and inform us, as we would expect, that the interstellar Nomads carry seeds of knowledge through the Third Dark Ages, enriching the later Galactic civilization.

Hloch of Stormgate Choth on Avalon edits the Earth Book and greatly increases our knowledge of its contents.

The Long Night collects:

the one story about the Founder of the Terran Empire;
one story set during the Flandry period of the Empire;
one set during the Long Night after the Empire;
one set during the subsequent Allied Planets period;
one set during the much later Commonalty period.

Miesel's comments make this volume read like a coherent overview of much of the Technic History. She refers to:

the seminal figures, van Rijn and Flandry, although neither appears in any of these works;

the colonized planet Vixen, invaded in Flandry's time, that founds its own colony, New Vixen, which in turn is flourishing millennia later when descendants of rebels expelled by Flandry return to civilization.

There is a sense of historical depth and continuity.