Dark Force Rising By Timothy Zahn

The dying Empire's most cunning and ruthless warlord—Grand Admiral Thrawn—has taken command of the remnants of the Imperial fleet and launched a massive campaign aimed at the New Republic's destruction. Meanwhile, Han and Lando Calrissian race against time to find proof of treason inside the highest Republic Council—only to discover instead a ghostly fleet of warships that could bring doom to their friends and victory to their enemies.

Yet most dangerous of all is a new Dark Jedi, risen from the ashes of a shrouded past, consumed by bitterness… and scheming to corrupt Luke Skywalker to the Dark Side.
 
- Goodreads Summary