An excellent collection of SFWA Grandmaster Damon Knight's always-excellent stories--three short novels in this case:

Rule Golden--what if the Golden Rule were enforced in reverse by some force--the bad things you do to others are done to you?

Natural State--are cities doomed by genetic engineering?

The Dying Man--what would it be like to be immortal... then suddenly find out you'd become mortal?

(Note that Rule Golden and Other Stories includes these same three, plus two more.)