Xizor (the green guy with the top notch) isn't trying to kill Vader just so he can become the Emperors right hand man as somebody here mentioned. Actually Darth Vader gave the order for some bio weapon to be tested and it nearly wiped out the whole civilization of Xizor's people. He was off planet at the time but his entire family died. He hated the empire and specifically Vader ever since then. What he is trying to do in the book is to kill Luke because he finds out he is Vaders son, and on top of that that Vader is trying to get Luke to join him. So he knows Luke is importand to Vader and he decides that this is a good way to get back at him.
I like the story because it deals more with the underground. Blacksun, whome Xizor is the head of, is a mofia sort of organization. The book also deals alot more with the imperial city, something I always wished would have been in the movies. Leia is captured and Luke, Lando, Dash and Chewie rescue her and blow up Xizor's head quarters. Then as they are escaping the planet Vader blows up Xizor's giant ship and they have to dodge their way through the peices, Dash gets hit by a big chuck a is killed (unless you beat the game on hard, then you learn he lives, for the game anyway).
I suggest skipping the book and checking the audiobook out from the library, I noticed it is one that most libraries seem to have for some reason. Also the comics are good, and the game is still one of my favorites.