But how does one learn, 30 to 40 years after-the-fact, which authors influenced which?

yeah, that's an issue. there's loads of history and commentary that still exists on influences in "textbook" literature, but most of the commentary on influences within the sci-fi/fantasy world tend to be in fan magazines long out of print. its all in existance somewhere, but not compiled in a way that will sell.

maybe wikipedia is one place where some people who have the original sources may be posting summaries. they've done it for other genres like movies and music...