Spelunky is a fascinating, deep game. Though I've played little of it, I appreciate from afar; I've heard several game designers espouse love for its brilliant mix of simple approaches to complex situations, and over the years I think the conversation on it has subsided, but never really gone away.
A new hour-long documentary from Noclip is as extensive a look at the game as we're likely to get. the film talks to the game's core designers, chronicles its history, and talks about its various layers and interactions in painstaking detail. It's long enough that you can even enjoy a few games of Spelunky as you listen to it in the background. You can find the documentary below.