Retro for 1939 (Best Related Work)
This is a recommend based on the idea that LonCon3 might possibly be running a Retro Hugo for 1939 .... and my recommendation, is for Best Related Work:
J.R.R. Tolkien _On Fairy Stories_.
"On Fairy Stories" was delivered as the Andrew Lang memorial lecture at St Andrews in 1939 (and was printed in 1947 in Essays Presented to Charles Williams, compiled by C. S. Lewis) - I'm not sure, therefore, whether it would qualify or not.
But if it does qualify, I can think of no better nominee for this category - its influential, its well written, its full of ideas.