"Shadows of the Empire" is a novel that bridges the gap between the events of "The Empire Strikes Back" and the opening of "Return of the Jedi". Han Solo has been frozen in carbonite and is being transported by the bounty hunter Boba Fett. Luke, Leia, and Lando Calrissian are following Fett in the Millenium Falcon with the hopes of rescuing their friend before he ends up in the hands of the gangster Jabba the Hutt. There is quite a bit more going on in "Shadows of the Empire" than just rescuing Han Solo (besides, we know from Jedi that Han will be delivered to Jabba's palace).
Prince Xizor is the head of the Black Sun criminal organization. Think of an extremely powerful and global mafia, expand it to a galactic scale, and that is Black Sun. Xizor is positioning himself as the top leader under Emperor Palpatine and directly in opposition to Darth Vader. This is something that cannot be done directly, because to openly oppose the favorite of Palpatine is a death sentence. Xizor discovers that Vader is instructed to bring Luke Skywalker to Palpatine, and after finding out that Skywalker is Vader's son, Xizor puts a death mark on Luke. If Vader fails to bring in Luke, and Xizor is not implicated, Xizor will rise in the favor of Emperor Palpatine. It is a fine line Xizor walks, but what else can be expected from the leader of Black Sun?
Leia becomes aware of the threat to Luke's life, and she works to save him (even without him knowing). She begins to contact Black Sun, always a risky venture, to learn what is happening and why. This brings Leia and the other Star Wars regulars (Chewie, Lando, the droids) to Coruscant and the heart of Black Sun.
Steve Perry has written a very good Star Wars novel. It ties in to the films, gives explanations for events in "Return of the Jedi" (ever wonder exactly why Leia is dressed as a bounty hunter?), and tells a compelling story in its own right. Xizor's plotting against Darth Vader is fascinating. This is old school Star Wars in the model (and era) of the Original Trilogy, and if that interests you this is the book to read.