Maybe Poul Anderson's four most important series are, in this order:
the History of Technic Civilization;
the Time Patrol series;
The King Of Ys (with Karen Anderson);
the
Harvest Of Stars Tetralogy.
The common theme is history, past or future:
the Technic History and the Tetralogy are future histories of different models;
the Time Patrol is a perfect synthesis of historical fiction with sf;
Ys is historical fiction with elements of fantasy.
The long novel,
The Boat Of A Million Years, is not on this list because it was not published as a series although, with its episodic structure, it could have been. Modern sf, e.g., Anderson's Time Patrol series, began as magazine stories or serials later collected in books, then progressed to original publication in books.
Boat also synthesizes historical fiction with sf but features immortals, not time travelers. These two groups have very different relationships to past times, e.g.:
the immortal, Hanno, lived through the entire nineteenth century but now, like everyone else, sees it receding into the past;
a time traveler can visit any part of the nineteenth century but does not live through all of it;
Time Patrolman Manse Everard visits the nineteenth century and could live through all of it because he has had longevity treatment!
The Time Patrol series never examines the full implications of
both time traveling
and living for an indefinitely extended lifespan. At least one more volume would have been necessary to address this and other issues like the real nature of the Danellians and the eventual fate of the Patrol, i.e., the members of the last class to graduate from the Academy must eventually die or retire from active duty? - at least in the timeline protected by the Patrol. There is always the possibility that a Patrol member traveling uptime arrives in a divergent timeline.
