The "Divided Dial" series (six parts) from WNYC's On the Media does an excellent job of detailing how and why radio in general (both AM & FM bands in the US) has come to be both so concentrated (in ownership) and divided (in content and audience). Both ... portend poorly.

The full series, with both audio and transcripts, is here: <https://www.wnycstudios.org/podcasts/otm/divided-dial>

Doc Searls, an old radio hand himself, often writes about the topic, though concentrating more on infrastructure than business or culture:

<https://blogs.harvard.edu/doc/category/broadcasting/>

I'd argue that AM radio, much as with FM and over-the-air television, is fading fast, especially in urban regions in which broadband Internet and cable programming offer far more attractive options. For myself, I find I'm listening to podcasts far more than informational radio (though I'll listen to some music programming) over the air --- it's convenient, and has very little advertising. Less than the nominally noncommercial public broadcasting alternative.

But in rural regions, where markets are sparse, AM's range still pulls in what few people there are, and motivated and ideological ownership often drives content, radio remains a potent force. The OTM series gives strong insights into those dynamics.

(The series was mentioned downthread by danielodievich: <https://news.ycombinator.com/item?id=34602095>)