No one but no one writes what William Gibson writes, nor writes how he writes. From the “Neuromancer” days, three and a half decades ago, we’ve been blessed with enigmatic, fast-paced science fiction (and sometimes almost not sci-fi) that vibrates with post-ironic intelligence. “Agency” follows 2014’s “Peripheral” in a universe where remnant oligarchs fiddle around divergent time-travel strands in the past, like Zeus’s menagerie of Gods. In the roaring opening pages of “Agency,” in a present-day America in which Trump didn’t win, our hero Verity, an app tester (of course) takes on a spunky new AI, Eunice by name, who struggles to understand herself even as the machinations of present and future battle over her. Gibson is a punctilious stylist who peppers his worlds with objects and settings almost, but not quite, normal, and he’s a super choreographer of his careening plots. Dialogue dominates even the most James-Bond-like scene, and the dialogue is waspish and beautiful to read. “Agency” is a monster of a read that may well be heralded as one of Gibson’s classics.