The Horn of Time by Poul Anderson (1968) is a collection of previously published science fiction short stories from the late 1950s and early 1960s. Nothing on the cover of the book hints that these are short stories and not a continuous novel so I spent some brain power for the first couple of stories trying to make them fit together into one universe before realizing my mistake.
I've always enjoyed the Anderson that I've read and, particularly for this era of science fiction, I think that the short story can be a much stronger vehicle than the full-length novel. Anderson is in top form here with stories that combine time periods, space travel, post-apocalyptic futures, and well written characters. My favorite of the bunch is probably the last story, "Progress," which takes us to a Earth that is healing from a long-ago nuclear destruction and running into conflicts as the now-powerful Maori people try to keep the other societies in balance and stop any chance of future nuclear wars.
Since these were written in the late-50s and early-60s there is some inescapable strong anti-communism running throughout several of the stories. In some cases this gets a little distracting (particularly in "The High Ones"), but it is a reflection of its time and its author, so what are you going to do.
Overall this is some solid and unique science fiction, and a must read if you are a fan of Anderson or the genre.