Poison Penmanship: The Gentle Art of Muckraking by Jessica Mitford
Part of what makes this collection so great are the personal comments at the end of each piece. There you'll find anecdotes on the writing process, things she wished she'd done differently, parts of the story that didn't make it in the final piece, and most enjoyable of all, behind the scenes looks at some of the more dubious characters she encounters. For the best laughs and enjoyment, try: Don't Call it Syphilis, Maine Chance Diary, My Short and Happy Life as a Distinguished Professor, and Let Us Now Appraise Famous Writers. For more somber and social justice-canted works, head for Trial by Headline and A Talk With George Jackson.
Incidentally, Mitford claims that her sister coined the term "frenemies" in what must have been the 1920s or 30s, a claim that could require some muckraking of its own