The Winter Of The World, Tau Zero and Time Patrol series.
In The Avatar, Earth is gradually approaching an Ice Age. Is this the Ice Age of The Winter Of The World? It could be but Anderson would have had to write a transitional volume to connect the two novels.
Both The Avatar and Tau Zero describe long interstellar journeys without a faster than light drive and within the laws of physics as understood at the time when the novel was written. The premise of Tau Zero is that a Bussard ramjet accelerates uncontrollably at relativistic speeds, thus traversing intergalactic distances in a very short time as experienced by its crew.
The premise of The Avatar is that the Others have placed a network of T machines throughout the universe. By approaching the T machine in the Solar System on a carefully controlled flight path, a spaceship can enter a target planetary system where it will be able to use another T machine to return home. Approaching a T machine on a random path can take you anywhere or when. Inevitably, an emergency will force the characters to the latter course - although I have yet to reread to that point so I look forward to being reminded of the details.
Random trips via T machines are the equivalent, in The Avatar, of the uncontrolled acceleration in Tau Zero. In each case, the novel's premise allows this kind of journey into the unknown so Anderson can explore the possibilities.
Imagine discussing as a matter of public policy whether to use a T machine to change history. Some Betans suggest preventing their scientific revolution but no path can be found to take a ship into their historical past. I think that that is how T machines work, allowing only consistent causality. If Betans intending to prevent a scientific revolution had emerged from their T machine earlier in their history, then Betan history would already have incorporated their arrival and its consequences and, if they had succeeded, then they themselves would have traveled back from a Beta without science so they would have had to use a spaceship borrowed from some other race. Thus, no Time Patrol is necessary.