(I see on wikipeedia that only a few of the Foundation books were written prior to SW, and that the same goes for the first two Dune novels. Would that be a good place to start?)
Dune would probably be a good place to go for some of Lucas’ literature inspirations. Star Wars was heavily based on ideas from Dune, and that’s why David Lynch’s Dune strayed from the book as much as it did, because if they included aspects from the book that were ever present in Star Wars, audiences that are unfamiliar with Dune (probably the majority of people) would feel it was a ripoff of Star Wars.