It was one of the publishing events of the millennium so far: the publication of Harper Lee’s second book in 2015, more than half a century after “To Kill a Mockingbird.” Actually, though it was set two decades after “Mockingbird,” “Go Set a Watchman” had been written before — it was the manuscript that piqued publishers’ interest in Lee and introduced Atticus Finch to the world. Lee’s intended follow-up to “Mockingbird” was to have staked out bold new ground. Conceived as a work of “In Cold Blood”-like reportage, its subject matter was no less hard-boiled — six corpses, a suspected serial killer rumored to dabble in voodoo, a vigilante who slew him and lawyer who represented both. Yet it was also familiar: an Alabama court case intersected by race — the alleged mass murderer and his avenger were black; their Atticus-like defender white. Moreover, Lee had furnished crucial assistance to her friend Truman Capote in the workup for “In Cold Blood” and gotten within one semester of a law degree before dropping out of college. The project seemed a match of author with subject. But it became an “albatross.” She finally abandoned and, with it, any aspiration to a post-”Mockingbird” second act. In “Furious Hours,” Casey Cep picks up where Lee left off with a full accounting of a shocking true-crime case and the great literary might-have-been it spawned.