When I first read Poul Anderson's Time Patrol series in the early 1960s, it was four stories in one collection, Guardians Of Time, and the collective villain was the Neldorians.
Time Patrol (New York, 2006):
collects ten stories, including the original four;
includes, among its six newer stories, two that are over 100 pages long, therefore counting as novels by my rule of thumb;
is the first of two Time Patrol volumes;
informs us on its back cover blurb that the collective villain is the Exaltationists - who, unlike the Neldorians, are continuing villains.
The second volume, The Shield Of Time, concludes the story of the Exaltationists but also reveals that there is a greater threat beyond them and that that greater threat is the real reason for the Patrol. Guardians Of Time and the Neldorians have been incorporated into the longer series but have also been left a long way behind - like the Torah incorporated into the Bible but also left way behind by the time the reader reaches the Apocalpyse.
We might discuss the Time Patrol blurb but not tonight.