Splinter Of The Mind's Eye by Alan Dean Foster

In this science fiction novel set in the Star Wars universe, Luke Skywalker and Princess Leia find themselves stranded on the fog-shrouded planet of Mimban while on a mission for the Rebel Alliance. As they navigate the planet's treacherous terrain, they discover the existence of a powerful crystal capable of amplifying the Force. Their quest to secure the crystal leads them into conflict with the Empire, culminating in a confrontation with Darth Vader. Throughout their journey, Luke and Leia must rely on their growing bond and resourcefulness to survive and protect the crystal from falling into the wrong hands.