Why the National Book Award for John Williams's August?

Though it's at times a little difficult to keep the cast of characters straight - w/ their unfamiliar Latinate names, such as Marcus Antonious (or something like that, rather than the more familiar Marc Antony) - made especially so by the back and forth between contemporary (43 BC) documents such as letters among the principals and "historical" documents, in particular a series of letters from 13 BC to the historian Livy presenting various characters' recollection of events - a little brush-up on Roman history will help keep things straight in John Williams's 1972 novel, Augustus - an account of the life and times for Octavious, later Augustus, Caesar, beginning the death of his uncle Julius and following through - in the first third of the novel at least, w/ many political and military machinations and plots to seize power, to form alliances (notably the triumvirate of Augustus, Marc Antony, and Lepidus) and to maintain power. As noted yesterday, it's both a contemporary novel - we can imagine the same kind of power machinations today in Washington yet the events also seem remote and even primitive: brutal hand--to-hand combat among the various Roman factions, a concord of Roman Senators assassinating the ruler who they believe has seized too much power (we haven't gone that far, yet), constant shifting of alliances among the powerful forces and their armies, blatant attempts to buy public support through distribution across the populace of wealth held in the treasury (or even private wealth of the emperors). Though many of the characters will be familiar at least in name to most readers, others are not, and I wonder which ones - particularly among Augustus' confidants - are from JW's imagination. I'm assuming that most or even all of the major events and battles are directly from Roman historians, Livy in particular. This novel must have entailed a lot of research, and I wonder what sent Williams down this course - so different from his earlier works, Stoner in particular. This novel won a National Book Award - as noted yesterday, the award should have gone to Stoner a few years earlier, so this is sort of a make-up; I think the NBA should go to works at least remotely on American themes, but at least in this instance the award recognized a great author who had lived and published in near-obscurity; he didn't much capitalize on this recognition, as A was his last book; I believe fro what I've read that he suffered from severe alcoholism and his writing essentially stopped w/ this novel, although he lived for another 20 years or so.