Shadows of the Empire By Steve Perry

Here is the untold story of the events that took place between the movies The Empire Strikes Back and Return of the Jedi: a novel in which Darth Vader still lives--and battles a villain as powerful and evil as himself.

SHADOWS OF THE EMPIRE

It is a time of crisis. Han Solo, frozen in carbonite, is being taken to the vile gangster Jabba the Hutt. As Princess Leia mounts a rescue mission including Luke Skywalker, Lando Calrissian, and a brilliant young pilot, Darth Vader pits himself against a cunning and ruthless rival. Xizor is the leader of a powerful crime syndicate who seeks to supplant Vader for the favor of their mutual master, the dreaded Emperor. Their target: Luke Skywalker. Suddenly Luke finds himself the potential prize of the two most evil entities in the galaxy--one who wants him alive...and one who wants him dead.
 
- Goodreads Summary