Shield

(Shield, not SHIELD: the latter means something else on the Comics Appreciation blog.)

I had remembered that Poul Anderson's Shield was set after a nuclear war but it is one of his futures in which such a war had occurred without destroying civilization. Society remains urban, even megalopolitan, with interplanetary capability.

Some standard sf props and familiar background details soon become evident:

the megalopolis is vast enough to join Boston, Norfolk and Pittsburgh;
there is a "Protectorate" to prevent further nuclear exchanges although this one is not global - China remains a rival power;
the reader must quickly ascertain whether for current narrative purposes the men from Military Security are to be regarded as good guys or bad guys;
"air cars" land on and take off from the flat roofs of tall city buildings, are assigned to various altitudes by a central "Control" and can be flown on autopilot (I previously mentioned such standardized futuristic vehicles in a list of familiar props when discussing Anderson's The Star Fox here).

I have reread only to page 14 of 158 so there will be more to say about Shield.