Splinter of the Mind's Eye by Alan Dean FosterThe first ever Star Wars expanded universe novel, released in 1978, this book was written under the requirement that it could be made into a cheap sequel to the first Star Wars film in case the movie didn't do well enough to justify a big-budget follow-up. It tells the story of Luke and Leia crash-landing on a swamp planet, where they must keep a powerful Force-magnifying crystal from falling into the hands of the Empire. Before the end, they'll have to face the Empire's most feared villain: Darth Vader.
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This is a classic era book from the Legends timeline. It will likely be contradicted by newer books (if so, just think of it as an alternate universe).