This is not so much a sustained historical argument as a series of reflections, primarily rooted in Sarris's reminiscences of roughly seven decades of film viewing and reviewing. Addressing broad categories (genres, directors, and actors), he zooms in for extended consideration of particular subjects (the Astaire-Rogers musicals, John Ford, and Vivien Leigh, among many others), creating intimately detailed miniature portraits that provide such studiously loving descriptions of classic scenes they may make the reader wish to hole up with a copy of the book and a VCR after having secured the services of a video store that makes deliveries. There is even a short final chapter in which Sarris discusses such "guilty pleasures" as My Foolish Heart, the only film ever made based on a J.D. Salinger story.
People who know movies, or think they do, will no doubt find something about which to disagree with Sarris. This is as it should be; "You Ain't Heard Nothin' Yet" is as much a commencement point as it is a summation.