Truman Capote's In Cold Blood is the ur-example of the genre. In November 1959, a family of four in western Kansas was brutally murdered in their home, and the lack of obvious suspects or an apparent motive perplexed local law enforcement for weeks. The perpetrators were identified and caught the following January. They were convicted after a brief trial, and after a five-year appeals process they were hanged.
Capote didn't invent the genre from nothing, but every piece of true crime feature writing since is descended from his book. Of course, Capote wrote a full-length book, rather than the magazine-length article one senses he would have written today. Would In Cold Blood be better if it were a lot shorter? That depends on what you're hoping to get from it. If you're used to reading modern-day feature-length crime journalism, and you're primarily interested in the details of the crime and the hunt for the killers, you're likely to find In Cold Blood to be padded and slightly tedious.
But Capote was able to write a full book where most writers would have stopped after a couple of thousand words because he delves deep into the psyches of the two perpetrators, Perry Smith and Dick Hickock. Capote got to know them both extremely well through visiting them in prison, and he treats us readers to lengthy descriptions of their formative experiences, personalities, and the psychology of their relationship. They, not the victims or the law enforcement officials, are the real stars of the book.
I've never seen the 1967 movie based on the book, but I have seen the 2005 film Capote, for which Philip Seymour Hoffman won an Oscar. Not quite the biopic it was advertised as, Capote is entirely about Capote's experiences writing In Cold Blood. More sardonic filmmakers would have given the movie Capote the same title as the book it's about -- the movie depicts Capote developing a severely coldblooded streak as he balances the close emotional relationship that's developed between him and Perry Smith, with the fact that he's going to have to see Smith and Hickock hang before he can finish his book. Excellent movie -- devastating portrayal of the man.
Capote keeps himself out of his own book, which never ventures into the first person. But it's quite fun to imagine what the juxtaposition must have looked like: these straight-laced, upright Midwestern farmers in 1959, and this man from the extreme opposite end of the American cultural spectrum who has come from New York to document them for posterity.