Is the genre of true crime having a moment, or is that moment centuries old? Likely both, I would say. With Truman Capote's In Cold Blood celebrating its half-century publication year, and appetites for criminal justice stories broadened by the podcasts "Serial" and "Criminal," the documentaries The Jinx and Making a Murderer, and FX's 10-part American Crime Story: The People vs. OJ Simpson miniseries, true crime talk no longer seems confined to furtive whispers in fear of snobby people judging mass-market paperback covers. Never mind many of those mass market books are really great, and the snobs deserve to be judged harshly.
Murder is, of course, an awful, ugly business, and Americans in particular continue to crane their necks to get a peek at the worst of the wrecks. We want answers, we want justice, we want heroes, we want villains, and real-life crime offers so many of these and then some. Crime fiction may desire order of out chaos, but true crime is making sense of the chaos and hoping for catharsis in the meantime.
This list is by no means comprehensive and definitive. (That's why you won't see super-obvious choices like The Stranger Beside Me or Helter Skelter or The Executioner's Song. I love those books but they're on all the other lists.) But these books stuck with me weeks or decades after I first read them. I suspect they will have the same impact on you, too.