It's also a landmark in novelistic reportage. It's been adapted twice, once as a movie and once as a TV miniseries; the events surrounding Capote's research into the facts have also spawned two movies, Capote and Infamous. Philip Seymour Hoffman won the Oscar for Best Actor for playing Capote in Capote.
In 1959 near Holcomb, Kansas, four members of the Clutter family (the father, mother, teen-aged son, and teen-aged daughter) were murdered by person or persons unknown. Capote was in the area within a couple of weeks to cover the investigation. The murders were brutal enough and mysterious enough to briefly spark national outrage.
The killers would turn out to be Dick Hickock and Perry Smith, two petty criminals who'd come up from Texas to the Clutter home because Hickock had been told stories by his cellmate during a recent prison stay that the Clutters kept all of their money at home in a safe. The Clutters did not actually do this. The small amount of money the two murderers got from the Clutter home was soon used up, though a couple of items taken by Smith would help clinch the case against them once they were apprehended.
The brutality of the murders and the subsequent revelation that they were essentially meaningless fascinated America for a time, especially once Smith and Hickock were caught several months after the Clutter massacre.
The stories and questions that swirl around the writing of In Cold Blood -- and specifically how involved Capote became with Smith and Hickock -- have come to obscure what a triumph the book is. Capote's vivid descriptions of place, character, and happenstance are marvelous and heart-breaking and occasionally sinister. He concisely presents the Clutters and their killers, the investigators and the neighbours, and a wide variety of other 'characters' both central and peripheral to the case.
If Capote had too much sympathy for Perry Smith in real life, it doesn't particularly show in the book: Smith is a fascinating charmer, but also a man capable of complete indifference to lives other than his own. It was Hickock who fantasized before the fact about slaughtering the Clutters, but it was Smith who actually did the killings: perversely, he did so after making three of the victims more comfortable and, in a paradoxical bit of humanity, preventing Hickock from raping the daughter before killing her.
Hindsight allows for certain new observations to make about Smith and Hickock. They both may have suffered from traumatic brain injuries as adults as a result of auto accidents. Hickock's obsession with killing the Clutters often presents itself as murderous envy spawned by a life of poverty and privation. Both Smith and Hickock seem to possess the uncanny charm often attributed to certain types of psychopaths. And so on, and so forth.
In Cold Blood does a lot of things very, very well. It's also a fine 'real-life' police procedural, and a fine 'real-life' court procedural. It's a testament to fine writing and reporting. Highly recommended.