Written in 1956, it was the first scifi novel ( at least that I read) that seemed to deal with adult themes. I don't mean erotics, I mean the kinds of issues that are beyond the sort of juveile space-opera that seemed to be the standard fare of the 40s and 50s. Without giving away the plot, it describes a civilization that, although decadent, has discovered the secret of immortality. But: it cant be given to everyone. A complex system of merit and accomplishment is set up. Advance up the ladder fast enough & you are given immortality. Fail, and you are euthanized. Of course, one strong individual challenges the system, etc. etc.
Brilliant!
--Michael Reynolds