Harvard philosopher Nelson Goodman wrote a series of articles in the 1970s on the subject of "about" (PROBLEMS AND PROJECTS). It was an interesting effort by a deeply gifted philosopher to get a handle on what a text is "about", without simply restating the content that the article or text includes. In hindsight Goodman's interest was prescient, and it points forward to current interest in the "semantic" web and search technology.
There is an interesting tool provided by Wordle that provides an intriguing way of extracting a sense of what a blog or other text is "about" by generating a word cloud based on the frequencies of words used. Included below are the results for a number of interesting blogs; I think they give a surprisingly nuanced picture of the main preoccupations of the bloggers.
It is Martha Stewart's blog!