Disclaimer: I've not specialised in media history, though I've been exploring it for a few years.
You can find clear bias through sources such as I.F. Stone (active 1950s -- 1970s), or George Seldes (1930s -- 1960s, lived to 1995). Earlier muckrakers included Upton Sinclair (The Jungle and It Can't Happen Here, 1910s -- 1950s), Ida M. Tarbell (The History of Standard Oil, 1890s -- 1920s), and Lincoln Steffans and Ray Stannard Baker (both late 19th century / early 20th).
There's a phenomenal interview with Stone from 1974 on ... Public Television's ... "Day at Night" programme: https://youtube.com/watch?v=qV3gO3zxQ1g
I've commented numerous times recommending Hamilton Holt's Commercialism and Journalism (1909), a short book highlighting the influence of advertising on media. It's short, readable, fact-filled, and available at the Internet Archive: https://archive.org/details/commercialismjou00holtuoft
Walter Lippmann's Public Opinion (1922) largely established the model for the "impartial press" (so called, only partially successful as such) during the bulk of the 20th century. It was based in significant part on Lippmann's own experience with failures of the news media in reporting accurately on the Russian Revolution of 1917--23.
https://archive.org/details/in.ernet.dli.2015.126489/page/n1...
Other authors I'd recommend would be Ed Herman and Noam Chomsky's Manufacturing Consent (the title itself is a reference to Lippmann's work), Robert W. McChesney's many books on media, Ben H. Bagdikian, The Media Monopoly (cited in the famouse Google "Backrub" paper (http://infolab.stanford.edu/~backrub/google.html), Neil Postman's Amusing Ourself to Death, and Edward Jay Espstein, News from Nowhere (https://archive.org/details/newsfromnowheret00epst).
The last looks at television news as of 1971, including the numerous biases built in to the medium through narrative focus, budget constraints, and technological limitations, as well as the pace and scale of operations. Not all of those still apply, though some do, but it does remain a persuasive argument of how even incidental characteristics can profoundly influence a medium.
For contemporaneous criticisms of television, see Edward J. Murrow's "Lights and Wires in a Box" speech, and Newton Minnow's "Vast Wasteland" speech, from the late 1950s / early 1960s.
https://www.rtdna.org/content/edward_r_murrow_s_1958_wires_l...
http://www.janda.org/b20/News%20articles/vastwastland.htm
I've compiled a few further references concerning media generally in a "light reading list": https://old.reddit.com/r/dredmorbius/comments/7k7l4m/media_a...