Between Volumes

Baen Books' The Technic Civilization Saga, compiled by Hank Davis, collects Poul Anderson's Technic Civilization future history series in seven volumes. Vols I-III encompass the Polesotechnic History period:

Vol I, The Van Rijn Method, separately introduces Nicholas van Rijn, David Falkayn and Adzel;

Vol II, David Falkayn: Star Trader, ends with the resolution of a potential conflict between van Rijn and his trade pioneer crew of Falkayn, Adzel and Chee Lan in "Lodestar";

Vol III, Rise Of The Terran Empire, begins with Mirkheim in which van Rijn reassembles the long disbanded team for one last mission.

What has happened between "Lodestar" and Mirkheim, Chapter I?

See:

Mirkheim, Prologue, Y Minus 9
Ages
Falkayn's Age
Falkayn's Age II
Falkayn's Age III
Falkayn Remembers...
Falkayn Remembers...II

What has happened? A lot.