The Star Wars universe was somewhat dormant in the early 1990s. It had been eight years since Return of the Jedi, with new material being mostly limited to the source books published by West End Games for the Star Wars roleplaying game.
Then, in 1991, came Heir to the Empire, the first book of Timothy Zahn’s Thrawn trilogy. Followed by Dark Force Rising in 1992 and The Last Command in 1993, Zahn reanimated the Star Wars universe. The books are very well written and a joy to read.
The Thrawn trilogy had a lasting impact on the Star Wars universe. It introduced several recurring characters (most important of them Mara Jade, Luke Skywalker’s future wife) and kick-started the New Republic era. As such it was the first true sequel to the original trilogy and defined the setting for other books. It also marked the beginning of renewed activity around Star Wars, starting with the X-Wing computer game in 1993 (the 1994 sequel TIE fighter featured Thrawn). A neverending stream of novels followed, and of course the prequel trilogy began in 1999.
The conclusion is simple: The three books of the Thrawn trilogy are a must-read (more than any other Star Wars books) for Star Wars fans.