Content analysis is one of the most common methods for comparing texts related to news, and it typically divides into two different approach. First, deductive (i.e., theory driven) approaches use predetermined codebooks to count differences across topics, sources, or audiences. Second, inductive (i.e., theory generating) approaches create codebooks through careful reading with the goal of producing qualitative interpretations.
Good sources on the more deductive approach include Nuendorf or Krippendorff, and for the more qualitative approach Schreier or Kuckartz