Dark Force Rising is the sequel to Heir to the Empire, it's also the first Star Wars book I ever read. Dark Force Rising was pretty nonsensical to me the first time I read it but improved tremendously once I had a context for who Mara Jade, Talon Karrde, and Grand Admiral Thrawn were. It's one of the best Star Wars books ever written and I encourage anyone with a love for the setting to read it.
The premise of Dark Force Rising is a mythical two hundred Dreadnaughts, lost like the Flying Dutchman, are within reach of the Empire. These two hundred ships are enough to change the balance of power in the galaxy and our heroes are, of course, anxious to keep them out of the hands of the Empire.
This book was made before the Star Wars galaxy was quite as big as it eventually became. In the movies, we only saw a fleet of about twenty-five Star Destroyers at Endor and the entire rebel fleet was overwhelmed by them. A rebel fleet, I point out, which is enough to cause the Empire considerable distress.
In the WEG RPGs, by contrast, there were 25,000 Star Destroyers spread throughout the Empire. With those kind of numbers, two hundred Dreadnaughts could be either a game changer or something that's nice to have but ultimately unimportant. I think the current Star Wars universe has grown to the point that two hundred Dreadnaughts would be considered little more than a drop in the bucket of the galaxy's military reserves but what do I know?
Dark Force Rising, like the Empire Strikes Back, is a good deal darker than Heir to the Empire. We get to meet the Noghri at length, who are basically a people inhabiting a Post-Apocalyptic Wasteland and who show just how easily it is to destroy a planet in Timothy Zahn's conception of the Star Wars universe. Admittedly, this is more "realistic" with how fragile eco-systems really are but it kind of makes the Death Star redundant. We also get to hear Joruus C'baoth's philosophy, which is about as Anti-Jedi as it can get.
As a digression, I'd like to point out I really didn't like what George Lucas did with the Jedi Knights. Back in the 1980s, I felt role-models were distinctly lacking for a lot of kids. George provided me a couple in Indiana Jones and Luke Skywalker, two fictional characters which helped shape who I am. By making the Jedi have feet of clay and lionizing sociopath basket case Anakin Skywalker in the Star Wars Prequels, I wonder if he unwittingly did kids a diservice.
Despite this, the Jedi Knighthood's philsophy remains one of humility and patience. Joruus C'baoth nicely shows a person doesn't have to be a cackling madman like the Emperor or Sauron to rip out the heart of what a Jedi is supposed to be. Instead, he just sets a Jedi Knight up as a superior being who lords over lesser people. It's particularly notable as Luke has come to Joruus in hopes of learning how the old Jedi dealt with having a position of respect and authority.
If only, in real life, our leaders approached their task with as much humility as Luke does. We'd live in a much better world.
The quest for redemption by Mara Jade continues in this book, following her as she's forced to choose between Talon Karrde and Grand Admiral Thrawn. A choice which, amusingly, Grand Admiral Thrawn doesn't really care to indulge. I found this to be an interesting dynamic between them, highlighting the subtle differences between the Empire as Mara Jade remembers it versus the Empire that Grand Admiral Thrawn is trying to build.
Back in the nineties, the Empire was still a stand-in for the Nazis and it's easy to see Mara as a sort of confused Post-War Hitler Youth. She's the kind of person who saw Palpatine as a godlike figure who served as her father substitute. Grand Admiral Thrawn is aware of this and seems to hold both Palpatine and Darth Vader in disdain, much like many German military officers considered Hitler and Himmler. If you object to my portrayal of Vader as Himmler, I agree, but it's the best analog I can think of.
(Bizarrely, I think of Vader as a Rommel analog. Which is amusing because Thrawn is ALSO Rommel.)
One of the interesting things that Prequel fans will note is that the book is strongly tied to the Clone Wars and Old Republic despite the fact George Lucas hadn't created either yet. This can lead to a lot of inconsistencies to fans determined to keep a straight continuity in their heads.
The Dark Force fleet, for example, is something that Lando Calrissian dreamed of searching for as a child despite the fact that the Republic didn't have a military less than twenty years prior. The character of Garm Bel Iblis is a legendary Corellian Senator seemingly from a different time when we know the Republic ended fairly amicably on the Senate's part. Finally, the heroes look at clones with a mixture of revulsion and horror when we know the Republic used them as opposed to fight against them.
Really, a running theme of Dark Force Rising is the futility of trying to recapture the past. Garm Bel Iblis and company are living in the days of the early rebellion, predating A New Hope. Mara Jade is living in the glories of Palpatine-era Imperial glory, probably equivalent to the heyday of Hitler's rise to power. Luke Skywalker is trying to look to the Jedi Masters of old for guidance when Obi Wan Kenobi obviously wanted him to found his own order with its own rules (even more obvious with the Prequels establishing Luke is not trained REMOTELY like other Jedi).
Even Grand Admiral Thrawn and Captain Pellaeon are living in a fantasy world constructed around rebuilding the Empire, when the entirety of the galaxy is just glad they're gone. Hell, it's questionable whether Joruus C'baoth realizes the Jedi Knighthood of old was nothing like he remembers it but he certainly wants to rebuild the organization. Only Princess Leia and a few others are looking to the future and it requires their efforts to liberate the galaxy from a (in-universe) crippling nostalgia.
Maybe I'm biased but Dark Force Rising is one of my all-time favorite Star Wars books. Everyone should read it who loves Star Wars.