All the King's Men is often described as being the story of a Huey Long type character (Willie Stark), who begins his political career as a populist who wants to help the people and ends it as a corrupt governor who will stop at nothing to consolidate his own power. But I see it as the story of how an everyman--Jack Burden, the book's narrator--can be corrupted by proximity to power. Jack's early life was rather aimless. He was a law student, a history student, and a reporter before he became Willie Stark's right-hand man, taking on any job from shepherding a tax bill through the legislature, to coordinating a photo op at the governor's family farm, to digging up dirt on a family friend who has been nothing but good to Jack.
The story of Jack's work with Willie Stark--"the Boss"--is interspersed with memories of Jack's youth--his romance with Anne Stanton and his friendship with her brother, Dr. Adam Stanton; his troubled relationship with his mother, who lives with a much younger man; his youthful and short-lived marriage; his research as a Ph.D. student in history (readers are treated to a long description of the Civil War era family he was hoping to write his dissertation on). Years after their romance ended, Jack learns that Anne Stanton is having an affair with "the Boss"; he is devastated--and it is the affair that sets off the events that result in the deaths of both "the Boss" and Dr. Stanton. In the wake of these events, Jack falls into a deep depression but eventually pulls himself out and sets his life on a more positive path--although I am unconvinced that he even then has taken full responsibility for his past actions.
All the King's Men won the Pulitzer Prize in 1947 and has appeared on numerous "best novels" lists. It certainly explores timeless themes--the corrupting effects of power, the consequences of not taking responsibility for one's actions, identity, and even original sin. For some readers, the poetic language (Warren was, after all, a poet) and style are probably moving; I found them ill suited to the book and quite often repetitious, pretentious, and annoying. An example: "There is nothing more alone than being in a car at night in the rain. I was in the car. And I was glad of it. Between one point on the map and another point on the map, there was the being alone in the car in the rain." And he continues in this vein to the point of tedium. For me, Jack was not redeemed by either his evolving life philosophy or his actions--indeed, he was a thoroughly repulsive character. Though I tend to blame myself when I don't care for a classic, I have to say I much preferred Ethan Canin's recent America, America, which shares structural and thematic similarities with All the King's Men but is set in the Vietnam era.
The best luck always happens to people who don't need it.