Besides the Flash Gordon serials, the original 1930s Flash Gordon comic strips by Alex Raymond are also well worth checking out if you want to look for the origins of Star Wars. There are a couple of different series collecting Raymond’s ten-year (1934-44) run easily available on Amazon. (A welcome improvement from when I started checking them out a few years ago, when they were much harder to come by!)

In literature terms, EE “Doc” Smith’s Lensman space-opera series contains a notable forerunner of the Jedi Order in SW, to say nothing of Frank Herbert’s Bene Gesserit. Edgar Rice Burrough’s John Carter series and Leigh Brackett’s Eric John Stark stories are also good examples of the planetary-romance SF which Lucas also read.

Also, don’t forget the films of Akira Kurosawa. (All of them. Any of them. But especially The Hidden Fortress and the Yojimbo duology.)