There's a book called The Media Monopoly that details how media companies have consolidated to just a handful of mega corps and the book had to be republished 5 times since the 80s because every few years the number keeps shrinking dramatically. The author later released a brand new book called The New Media Monopoly which is essentially the 7th edition of the original book and at this point we're in a fucked up late stage version of the problem he originally detailed.
From the Wiki on the author:
In 2000 Bagdikian stated, "Every edition has been considered by some to be alarmist and every edition ends up being too conservative." In this latest version, Bagdikian wrote that the number of corporations controlling most of the media decreased to five: Disney, News Corporation, Time Warner, Viacom, and Bertelsmann. He argued, "This gives each of the five corporations and their leaders more communications power than was exercised by any despot or dictatorship in history."