This is one of my favorite Star Wars novels. In some ways it's a sequel to Children of the Jedi, though written by a different author. I loved the Luke and Callista storyline. Highly recommended.
Luke Skywalker's quest to restore the Jedi powers of his beloved Callista leads him deep within the Core. He finds there that even though the Empire is but a fragment of itself, yet another mechanical dreadnought is under construction. It promises to be more terrible than previous undertakings, but due to limited resources, the project is in as much danger as unarmored stormtrooper boarding a spice smuggler's cargo ship.