Star Wars: The Hand of Thrawn Duology #1: Specter of the Past by Timothy Zahn

The Empire stands at the brink of total collapse. But they have saved their most heinous plan for last. First a plot is hatched that could destroy the New Republic in a bloodbath of genocide and civil war. Then comes the shocking news that Grand Admiral Thrawn–the most cunning and ruthless warlord in history–has apparently returned from the dead to lead the Empire to a long-prophesied victory. Facing incredible odds, Han and Leia begin a desperate race against time to prevent the New Republic from unraveling in the face of two inexplicable threats–one from within and one from without. Meanwhile, Luke teams up with Mara Jade, using the Force to track down a mysterious pirate ship with a crew of clones. Yet, perhaps most dangerous of all, are those who lurk in the shadows, orchestrating a dark plan that will turn the New Republic and the Empire into their playthings.