The first Star Wars novel to be set after Return of the Jedi, Timothy Zahn’s Heir to the Empire was released in hardcover on May 1, 1991. It was the first major step out of what’s now called ‘the Dark Times,’ when Star Wars was (mostly) a dormant franchise. The book was immensely successful and helped prove there was still an audience for Star Wars
Although now classified as non-canon ‘Legends’, along with the rest of the pre-2014 Expanded Universe, Heir remains an important part of franchise and fandom history. Hell, we wouldn’t exist without it.
Also it’s a damn good story and you should read it (along with ’92’s Dark Force Rising and ’93’s The Last Command) if you haven’t already. Though note I am totally biased.