I think I'm going to choose End of Eternity
as my next book, I've not read it in decades... Gods Themselves
is a cool one as well, and a nice commentary on the workings of science. It also has his only alien race [as opposed to the collective intelligences of Gaia and Erythro] and a pretty neat one I think. It might have been interesting if he'd done more with aliens, but even with his output there wasn't time for more than humans and robots!