I never read the book... but I think it's a jungle planet and he faces Vader in a cave or something... I know that's not much help.
This comic, if you could find it, might be useful.
Here is a summary of the story -Luke Skywalker and Princess Leia are traveling to Circarpous IV for a Rebel underground meeting in an attempt to take the Circarpousians into the Rebel Alliance. Leia's ship experiences problems; she is forced to land on Circarpous V, Mimban, a foggy, swampy planet. Luke follows. A strange colorful energy storm caused by energy mining forces their ships down in a crash landing.
Luke exits his X-Wing and, with R2-D2, begins searching for Leia's Y-Wing. He finds it; Leia is there as well as C-3PO. They all begin walking in search of a beacon that could possibly be a station that would allow them to get off the planet.
Instead, they find a mining town; Leia sees that the Empire has a secret energy mine here. They enter unnoticed, steal some mining clothes, change into them, and enter a bar. An Imperial suspects, so he investigates, and to cover up Luke and Leia's true identities, Luke claims that Leia is his servant girl. The Imperials no longer suspect. An old woman named Halla comes over and quietly talks to Luke and Leia; she identifies Luke as one who is strong with the Force. Then the old woman takes out a splinter of a glowing crystal that she identifies as the Kaiburr crystal, which to Luke seems to magnify and focus the Force to a Force-sensitive. She has but a shard of the crystal; she is in search of the whole thing. She strikes a deal with Luke and Leia that if they help Halla find the Kaiburr crystal, Halla will help Luke and Leia get off the planet. They all exit together.
Upon emerging, Leia admonishes Luke for slapping her in the restaurant and calling her his servant girl; Luke falls off the walkway and into the mud. He pulls Leia down with him. They begin playfully fighting as some miners emerge from the building. They claim that fighting in public is against Imperial law here, so they all get into a brawl.
Imperial stormtroopers bring everyone, except for Halla, C-3P0, and R2-D2, who all hid in the shadows, to the local jail. They are all questioned by Captain-Supervisor Grammel. The miners who confronted Luke and Leia are taken away while Grammel continues questioning with Luke and Leia. Grammel discovers and confiscates the crystal shard after having already confiscated Luke's weapons, most prominently his lightsaber. Luke and Leia are placed in the maximum security cell with two drunken, but friendly to Luke, Yuzzem named Hin and Kee, while Captain-Supervisor Grammel reports the incident and the crystal shard to the region's governor, Governor Essada.
Halla shows up in the jail cell's window. She, having claimed to be a master of the Force, uses the Force to lift up the food tray through the jail bars and over to the open/close mechanism so that the jail cell opens. The two large, hairy, and muscular Yuzzem go on a rampage killing Imperials while Luke and Leia make their way to the exit. The two Yuzzem catch up with Luke and Leia as they all exit the jail together; the Yuzzem give Luke and Leia their weapons.
The four make their way to a pre-specified meeting point. They find Halla there, who steals a vehicle. They all begin making their way to the place where Halla believes the Kaiburr crystal to be, the Temple of Pomojema. As they are driving through the swamplands with little pursuit due to Luke's quick thinking. They encounter a large, pale white worm with huge jaws. It begins chasing them; eventually they all must exit the vehicle. Luke and Leia split up with Halla, C-3P0, R2-D2, Hin, and Kee; Halla's group loses the wandrella creature, while Luke and Leia are still being chased.
Luke and Leia find a deep well into which they climb down vines and hide on a ledge. The wandrella finds them but falls down the well in an attempt to get to them. Its massive body takes the climbing vines that acted as ladders with it. Luke and Leia wait for some time until Halla's group finds them. There is no way back up, so Luke and Leia are told by Halla that there should be a passageway back up to the surface in the back of the ledge on which they are sitting. Halla's group leaves to make their way to the ground-level end of the passageway while Luke and Leia journey through it.
Luke and Leia encounter a lake, onto which they must float using a lily pad–like boat; they encounter another creature in the black waters of the underground lake, but Luke makes it go away with his father's lightsaber. They get to an abandoned underground city where they are stalked and attacked by some Coway guards/patrols. All but one Coway are dispatched; the one goes off and tells his Coway tribe about the visitors. Luke and Leia follow the single Coway to the tribe.
There, Luke and Leia see that Halla's group made it to the other side of the passageway and had already made their way into it to meet up with Luke and Leia, but here they have been tied up by the Coway. To save his friends, Luke must fight the Coway's champion fighter; Luke wins. The Coway then become friends and feast with Luke, Leia, Halla, Hin, and Kee. But Luke, in the Force, senses Darth Vader. Coway patrols race over to the tribe and confirm Luke's feeling: Imperials, led by Darth Vader and Captain-Supervisor Grammel, are attacking the underground cave.
The Coway tribe and the protagonists prepare for the attack. The Imperials arrive, but they are surprised by the Coway tribe's powerful response and the Imperials face a debacle. Darth Vader and Captain-Supervisor Grammel retreat with very few surviving stormtroopers. Darth Vader and Grammel leave in search of the Temple of Pomojema while Luke and company steal an Imperial transport left behind. They travel to the temple as well.
Upon arriving at the temple, Luke, Leia, Halla, Hin, and Kee exit to find the Kaiburr crystal and the fact that they have beaten Grammel and Vader to the temple. They encounter yet another creature with large jaws; they try to fight it off with ineffectual blasters. Luke tells Hin and Kee to go get some rifles. Luke finally decides that cutting down one of the pillars holding up the temple will crush the creature; it does. They all then make their way back to the Kaiburr crystal located in the back of the temple.
The two Yuzzem, Hin and Kee, never return. Luke wonders why, but then is crushed by rubble that fell from the cracking, ancient ceiling. Darth Vader then enters the Temple of Pomojema after having turned off the droids and killed Captain-Supervisor Grammel, Kee, and supposedly Hin. Leia takes up Luke's lightsaber and begins fighting Darth Vader, with little success; she is badly injured. Hin, badly injured, shows up and lifts the big rock off of Luke's leg. Luke then fights Darth Vader until Vader is supposedly about to win, but instead falls into a pit; Luke senses that this does not kill Vader. As the story ends, a healed Leia and Luke drive off with their droids and with Halla into the mists of Mimban, ready to take on further off-world adventure.
Hope that helps some Hemble.