The Thrawn Trilogy by Timothy Zahn
Among the most beloved of all Star Wars novels, the so-called Thrawn Trilogy (a name that doesn't actually appear on the books themselves) was the story that launched the Legends expanded universe in earnest back in 1991. These books were the first ever written to take place after Return of the Jedi, and they set the stage for much of the rest of the Legends continuity. They tell a masterfully-written, multi-faceted story that's at once both epic and deeply personal.
Five years after the destruction of the second Death Star, the budding New Republic is threatened by Grand Admiral Thrawn, an alien military genius who has returned from the Unknown Regions to retake the galaxy for the Empire. Worse, Thrawn teams up with an insane Jedi clone who boasts access to secret Imperial technologies. Luke Skywalker, Han Solo, and Princess Leia must rediscover old secrets and search out new allies in the struggle to stop Thrawn. But among these new "friends" is Mara Jade, a former Imperial agent who lives for one purpose: to kill Luke Skywalker.
The Hand of Thrawn duology is a sequel to this trilogy, set ten years afterwards.
Buy on Amazon:
Book 1: Heir to the Empire
Book 2: Dark Force Rising
Book 3: The Last Command
This is a post-Return of the Jedi series from the Legends timeline. Think of it as an alternate universe from The Force Awakens and newer books.
(Do you miss Grand Admiral Thrawn? He's back in the new Star Wars canon, starting with Timothy Zahn's novel Thrawn!)