Here’s a list of books that cinema scholars and Lucas himself cite as influences on Star Wars:
Isaac Asimov’s Foundation Series: starting as a series of short stories published between 1942 and 1950, Foundation features a Galactic Empire very similar to the one depicted in the original Star Wars trilogy. There are even characters named Han and Bail.
The Hobbit and The Lord of the Rings: besides Obi-Wan Kenobi filling much the same role in A New Hope that Gandalf did in The Fellowship of the Ring, early drafts of Star Wars had an even closer resemblance to Tolkien’s beloved tales. At one point, Lucas toyed with the idea of casting dwarfs as his main characters.
Arthurian Legend: there are many parallels between Luke Skywalker and King Arthur. Both Obi-Wan and Yoda resemble Merlin in several respects. Anakin Skywalker shares much in common with Uther Pendragon.
Frank Herbert’s Dune: the most frequently used setting in Star Wars is a desert planet. There are multiple mentions of spice, and many Jedi powers are similar to Bene Gesserit techniques. Herbert himself pointed out 37 direct Dune references in Star Wars.
Jack Kirby’s Fourth World: the original run of Kirby’s New Gods stories was published by DC Comics from 1970–1973. A major theme of the Fourth World comics is a hero destined to defeat an evil tyrant who turns out to be said hero’s father. Roy Thomas, then an editor at Marvel, allegedly pointed out similarities between Kirby’s series and an early Star Wars synopsis during a 1972 dinner with Lucas.
The Hero With a Thousand Faces: Lucas’ appreciation for folklorist Joseph Campbell’s seminal book is well known. Campbell’s treatments of monomyth and the Hero’s Journey are baked into the themes and plot structure of Star Wars.
Gone with the Wind: seriously. Watch The Empire Strikes Back and pay attention to Han and Leia’s dialogue.
Since Leigh Brackett wrote the first draft of the Empire screenplay, I might suggest one of her movies, like The Big Sleep (1945), Rio Bravo (1959), or The Long Goodbye (1973).
If you include tv shows, don’t forget “Star Blazers.”
I was very disappointed the the link to Frank Herbert’s claim of 37 similarities of Dune to Star Wars did not actually have a list. This link has a table with fifteen good ones:
http://www.moongadget.com/origins/dune.html