The big question in the non-fiction novel In Cold Blood by Truman Capote is why murder a family of four people, why invade their home in the middle of the night, tie the up, gag them, and shoot them in the face with a shotgun? The answer is fairly simple, actually; the murderers were psychopaths, meaning they had no conscience and no attachment to life, to other humans, or even to their own life. But it doesn't seem obvious to the neighbors of the victims, to the investigators, or even to the reader. It seems senseless and horrific, and you have to wonder how anyone's personality could become so warped that they would commit mass murder.
Capote developed several innovations in order to consider this problem. In the first place, there's is no suspense in the usual sense: Capote used a famous murder case in which the perpetrators had already been convicted and hanged by the time the book was published. But for the detectives there is plenty of suspense; they have a real "who done it" with very few clues. Capote develops the characters of a few of the detectives, and follows them in their investigations, so that the reader feels their suspense. There is also plenty of suspense for the people who knew the murder victims—the prosperous Clutter family—as they all begin to suspect each other, and to be fearful about the future. The characters of a few of these people are sketched in as well, so the reader absorbs their suspense.
The innovation that set the literary world abuzz was that the novel occupies a new space in between factual reporting and fiction. Capote conscientiously reports every scrap of evidence, even false leads, what would be 'blue herrings' in a British mystery show. He reports every interview. He quotes from the detectives' notes. It's a wonder that he can keep the reader interested in all this minutia. But he escapes the bounds of journalism by using the facts to conjure scenes, complete with atmospheric details and dialog that the author could not have heard. Of course, some people object with his taking this much license, but that's what makes it a novel; otherwise, it would just be journalism, and nothing new in terms of form. Also, it would not have become a major hit in the publishing world. People were already familiar with the basic facts of the case; it is the imagined part that holds your attention.
A third quality of the novel is harder to describe. Usually the narrator of a novel has a consistent voice, a sort of attitude, a point of view. In Cold Blood is presented in the form of reports from the participants, so the point of view is constantly shifting. Part of the story—part of the evidence—is told from one character's point of view, part from another, following several characters. The reader hardly notices any general narration pulling all these "factual" reports together.
Instead of a voice, the novel has style, a truly exemplary style of using the English language. Capote's sentences are remarkably clear and graceful—just the right choice of words, the perfect word order, no clutter or self-consciousness. Each sentence calmly, sensibly, sympathetically leads to the next.
The reader experiences the magnitude of the crime—we meet the victims and hear their dying words as they are slaughtered methodically. We experience the gnawing hunger for answers that motivates the lead detectives in the case. But most importantly, we get an intimate look at the childhood and development of psychopaths, not just the two murderers in this case, but of other mass murderers as well, plus the text-book description of this pathology. And the result is the reader feels some empathy with them; you can see how a neglected and abused little kid could become angry at the world; you can feel how much they needed a little love and guidance when they were vulnerable. People aren't born psychopaths; it's a result of the way they are treated in those early years; maltreatment warps their personalities. Which is not to say you sympathize with them. No, hanging seems the right punishment in the situation. They seem irredeemable. They don't even care. They place no value on their own lives, never did.
