Shadows of the Empire by Steve PerryThe only novel set between The Empire Strikes Back and Return of the Jedi, this book was published in 1996 as a refreshing return to the classic trilogy era, in the years where most novels took place after the defeat of the Emperor. The book is essentially a lead-in to Return of the Jedi, setting up the background for many of the events in that movie. However, it revolves heavily around a new character, the crime kingpin Prince Xizor, who is competing with Vader for the Emperor's favor.
Though the book tells a reasonably engaging story, many readers remember it more for its creepy attempted rape scene (for the record, most Star Wars novels have no sexual content at all).
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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).