Walker, a British film critic and historian, has thumbed through Variety and the other trade papers from mid-1926 to late 1929; he's seen the primitive ""speakers"" they report on; and, from this double exposure, he has some corrections to make in the record as well as some fresh thoughts on the whole silent-to-sound transformation. So the film-history fraternity will take note. But by choosing to chronicle the developments of these two-and-a-half ""revolutionary"" years virtually month-by-month, he's produced a narrative that reads like a combination film chronology and fever chart: each new release elicits cheers or jeers, raises or dashes hopes, promotes the talkies or triggers a backlash. And certain themes -- the silent stars' ""loss of divinity"" when first they speak, the non-performance of polished stage performers--recur without variation time and again. Still, Walker does record both the novelties that kept public interest in the talkies alive--from variety-show shorts, to the ""first all-talking"" film, to the wave of Hollywood musicals--and such real advances in sound-film technique as the traveling microphone boom, prescoring and playback, and the mix of dialogue and silent footage (too much talk was one early complaint). Along revisionist lines, he contends that it wasn't Jolson's blurting out his catchline ""You ain't heard nothing yet"" that was the breakthrough in The Jazz Singer--""filmgoers in 1927 would have expected this very line from Jolson""--but the film's other spoken lines, his ""naturalistic,"" ""emotion-charged"" exchange with his doting mother. And poor John Gilbert, he claims, wasn't ruined because he was given ultra-romantic lines that embarrassed the public. A plausible argument, and germane to Walker's attempt to rebut the alleged ""mass carnage"" of poor-talking silent stars. But only at the close does he inject his most interesting idea--that the coming of the talkies made U.S. films American in subject and texture (and the record backs him up). The fullest treatment of the changeover yet--variously tedious, stimulating, and instructive.