What is a Golden Age classic?

I found this nice definition here:

During roughly the 1920s and 1930s, between the World Wars, the British detective novel flourished and set the standard for its type. These books were meant to be entertainments, games where the reader matched wits with the author, so their hallmarks were cleverness of murder and detection methods, graphic violence or sociological comment kept to a minimum, stylish writing, and a satisfactory conclusion where order was restored to the community by an essentially honorable detective to confirm the reader's notion that the English way of life was the best on offer.

The modern reader, looking for an escape from a world where social and political problems seem insurmountable, can be just as entertained by the plots and characters and also by the glimpse into the customs and attitudes of a by-gone era.

That last part is right on!