In the days before video, if you could get access to twenty or thirty films and come up with a few insights, you could get a book published. This book, written originally for a thesis or dissertation in American studies, is one of those. Occasionally, he comes up with an accurate observation, but much of what he says is based on a very limited familiarity with films of the era. Indeed, most films fans of today, with access to video and classic film stations, will have seen more than Bergman had when he wrote the book.
He follows the traditional line: Mae West was the most incendiary thing on the thirties screen (hardly); the Marx Brothers' DUCK SOUP was daringly political and that's why it was unpopular (not true.) If he'd seen other films about sex besides Mae West's -- or had seen enough films to realize just how political movies could be in the early thirties (when Duck Soup was made), he'd not have jumped to those conclusions. But you can't be an expert by leaping to lofty generalizations based on limited knowledge.
His analysis of the film FAITHLESS, for example, proceeds from a complete misunderstanding of its context. He talks about a daring, feminist film as though it were a throwback because he has no idea of what he's seeing.
Thirty years later, Bergman probably knows better, but the idea of anybody reading this book and thinking they're getting some kind of real knowledge about the subject is saddening. I'd be willing to bet that ninety percent of the people who read this know more about the subject than Bergman did when he wrote about it.