In 1988, Marvin Olasky wrote Prodigal Press: Confronting the Anti-Christian Bias of the American News Media. An updated edition, including a co-author, Warren Cole Smith, was published in 2013. Here is an interview (video, 7:38, October 29, 2013) with that co-author.
This bias is, of course, obvious. Such books are useful, however, because they gather and document the evidence for saying so.