The only decent one from the 30s

They follow the plot almost exactly. In the book there is an ambiguity about the dog, and the reader is not quite sure what happened in one of the murders. That is made more explicit in the movie, but it does not change the story much.

You do have to pay attention, as there is not much exposition - but it is worth the trouble. All the other Mason films from this period are just jokey programmers. But see this one.