In the far future, Earth has become a university planet with Time University and Supernatural University among the schools. Time travel is a simple matter, as is instantaneous transportation between the stars. There's a slight problem for a professor at Supernatural, however. When he returns from a trip to a distant galaxy, he discovers that he's already returned a couple of weeks earlier and been killed. And if that's not complication enough, the planet to which he traveled isn't the one he'd been sent to. It's a strange crystal world that is offering to sell knowledge far in advance of that which Earth has already, for a price. But what's the price?
All this is happening on an Earth where fairies, goblins, trolls, neanderthals, ghosts, banshees, and humans all live and interact together. It's all told in Simak's trademark bucolic style.
The novel was nominated for a Hugo long ago, and I enjoyed it, but it's not one of my favorites of Simak's work. I can't recommend that you run out and buy a copy, but if you like Simak and haven't read it, it's certainly worth a look.