The Thrawn Trilogy consists of 3 novels: Heir to the Empire, Dark Force Rising, and The Last Command. The books tell a story that takes place in a galaxy far away, a long time ago. But it is not as long ago as the events of the 6 Star Wars movies. The story begins a few years after the the Rebel victory at the Battle of Endor at the conclusion of Return of the Jedi. Although the Endor conflict and its aftermath left the Empire weakened and resulted in the deaths of Emperor Palpatine and Darth Vader, The Empire still maintains a formidable navy commanded by Grand Admiral Thrawn, a brilliant military tactician.
Han Solo and Princess Leia are now married and Leia is pregnant with their twins. Thrawn, in his quest to re-establish the Empire, enlists the services of Joruus C'baoth, the insane clone of a dead Jedi master, who is intent on enslaving Luke, Leia, and Leia's twins (the last of the Jedi line) and establishing himself as galactic dictator.
The rebels match wits with Thrawn across multiple planets, the battles flowing in favor of each in turn. Both sides try to enlist the help of neutral parties, such as a guild of smugglers led by Talon Karrde and the Noghri, a race tricked into decades of servitude by Darth Vader and the Empire. In the final book, our heroic rebels face off directly against Thrawn and C'baoth for control of the known galaxy.
These books are part of the Star Wars Expanded Universe (EU), the name given to books, games, and other media that describe the events of the Star Wars universe. Although LucasFilms recently announced that expanded universe stories are no longer considered canon, it is still fun to follow the further adventures of Luke, Leia, Han, Lando, C-3PO, and R2-D2. Zahn stays true to the characters created by Lucas. He also introduces fascinating new characters, such as Karrde, Thrawn, Mara Jade, and others, who appear in later EU stories.
Along the way, Zahn weaves an exciting tale of action and intrigue, loyalty, and betrayal. It's not the most intellectually stimulating tale but it is loads of fun. Like an unexpected visit from old freinds.