Arnold Bennett was the most successful writer in English in the first quarter of the twentieth century, easily outselling his main rivals, Wells & Kipling. I suspect the reason for the relative decline of his reputation is that he his work has not adapted to film or changing times as well as the writers just mentioned. He writes about ordinary events,uses the slightly facetious tone characteristic of his period, and the narrative voice is more important, thus being less easily replaced by camera & microphone. 'Mr Prohack' was a good novel, but not at the top of his body of work.
Nevertheless 'Dear Mr. Prohack' is a solidly watchable film. Cecil Parker's dry presentation is reasonably close to that of Bennett himself and the film comes as close as possible to reproducing the authorial voice of Bennett. Unfortunately that is not very close. Bennett was a novelist, not a screenwriter. Well worth watching for all that.