Chuck Wendig's Aftermath books act as a sort of in-between trilogy, bridging a lot of the gap between the original Star Wars trilogy and The Force Awakens. Each book has been better than the one before it, and Empire's End is the best by far. It also brings a lot of changes and additions to the new canon. Here's everything we learned about a galaxy far, far away.
Image: Cover of Aftermath: Empire's End, Del Rey
Note: I'm not going to get into the plot minutiae here. You can and should read the book for that. (Especially since every character is a treasure trove of fun lines and the end is a great action piece.) That said, there are still a few things that might be deemed spoilers, so...
The Emperor Had a Plan
I cannot emphasise enough how much more bananas Emperor Palpatine was than we thought. He didn't just found the Empire as a way to give himself powerrrrr, unlimited powerrrr, he also fully believed that the Empire should not continue on without him. His philosophy was basically the opposite of "the captain goes down with the ship." It was more "if the captain goes down, make sure the ship and everyone on it is blown to pieces for letting the captain die." Palpatine gave Aftermath's main Imperial antagonist Gallius Rax the "blowing the Empire to pieces" job. (Also, definitely read this book to get Palpatine's views on chess. Spoiler alert: Letting the king go down is the worst failure possible)
So while the Rebellion and Luke Skywalker's actions at the Battle of Endor are important, the Empire's dissolution was hastened by Palpatine's own design. Also, part of his plan was to figure out calculations to leave this galaxy and head out to another. Those calculations finished after Palpatine died, but that's where his Super Star Destroyer ended up, and it's probably where a lot of the First Order was created. Go watch Return of the Jedi again with that knowledge rattling around your brain.
Image: The Force Awakens, Lucasfilm/Disney
Jakku Is Much More Interesting Than You Thought
Speaking of secret Palpatine plans, the desert world that Rey found herself on is so much more interesting than what we saw in The Force Awakens. For one thing, it used to be covered in water and plant life. At some point, it changed into desert, but with a "spark" of life still hidden in its core.
Jakku was also one of several worlds that the Emperor established "Observatories" on. An observatory held any number of things the Emperor thought needed to be hoarded: Sith artifacts, weapons, prisons, etc. They all have replicas of the Emperor's flagship and sentinel robots programmed to act like the Emperor and have his face projected on them, which sounds incredibly creepy. Jakku's Observatory was built as part of Palpatine's "in the event of my death" plan. Also, the Observatories can destroy the planet its on; Jakku narrowly avoids this fate.
Jakku is the home planet of Gallius Rax, who was originally an orphan in the care of a religious order on the planet, and taking the vows made one an "Anchorite." Kids were left to be raised with the Anchorites, who are very harsh in their expectations.
At one point, Norra Wexley is in a work camp on Jakku where her rations are described almost identically to what we saw Rey eating in The Force Awakens ("a ballooning piece of bread"). It's minorly interesting that the same rations that Norra was given will also be what Rey depends on to live.
Also, if you want to know how epic a battle has to be to justify why people are still combing Jakku to salvage parts in The Force Awakens, this book gives it to you. And during the battle, Wedge Antilles mentions the Tierfon Yellow Aces, which is nice continuity since the helmet Rey has in her little AT-AT home in The Force Awakens belonged to one of their pilots.
Image: Temmin "Snap" Wexley, Lucasfilm
Speaking of the battle of Jakku, Snap Wexley (played by Greg Grunberg in The Force Awakens) flies with Wedge's Phantom Squadron in the battle of Jakku and crashes on the planet. I like to imagine Snap and Rey comparing notes on the sandiness of the planet.
Snap also ends this book a) growing some scruff, like he sports in The Force Awakens b) heading to the New Republic flight academy on Hosnian Prime c) doing that with the man in charge being Wedge Antilles, who is basically his stepfather at this point. I really hope Wedge wasn't still on that planet when it exploded in The Force Awakens.
With the Empire Gone, Crime Has Flourished
It turns out that having the two biggest power centres in the galaxy focused on each other is a good thing for crime syndicates. So good for them that they do not want the war to end. To that end, the Black Sun crime syndicate -- one of the things invented in the old Expanded Universe that's made it back into canon in a big way -- and the Red Key Raiders (a crime syndicate out of Tatooine) actually work together to manipulate the New Republic into dragging out the war.
The idea basically is that, once the Empire is taken care of, the New Republic will turn its eyes on slavers, smugglers, gun runners, etc. The longer the war goes on for the Republic, the better for the criminals
The war was also made it possible for pirates to get their hands on Imperial military hardware. Eleodie Maracavanya returns in Empire's End, a character who appeared in Aftermath: Life Debt just beginning to consolidate her power as "the pirate ruler of Wild Space." Here, Maracavanya orders her fleet to attack Imperial ships fleeing Jakku, promising to send the bill for this "clean-up" to the New Republic. There are so many disparate power bases in different places in the Star Wars Universe, and these criminal groups are just a few examples.
Image: The Force Awakens, Lucasfilm/Disney
There Are Force Cults Everywhere
One interlude in this book tracks the attempt of a group of pilgrims to Christophsis, a planet where the Seperatists clashed with the Republic during the Clone Wars (and which appeared in Clone Wars TV series). Christophsis has a lot of crystals, and, apparently, it is one of the planets where the Force-sensitive Kyber crystals can be found. Many crystals there were taken to be used on the Death Star, but now pilgrims of the Church of the Force are returning them to the place they came from. The Church of the Force has access to the Journal of the Whills, which poetically describes the Force and the Jedi. Part of it was transcribed in The Force Awakens novelization, but we get more of it in this book (the last line may sound a bit familiar):
The truth in our soul
Is that nothing is true.
The question of life
Is what then do we do?
The burden is ours
To penance, we hew.
The Force binds us all
From a certain point of view.
In addition to a church devoted to the Light Side of the Force, we also have groups on the Dark Side, which all seem to be related to/predecessors of Kylo Ren's Knights of Ren.
One of these is the Acolytes of the Beyond, one cell of which is on the planet Devaron. Like the Church of the Force, the Acolytes believe everyone is connected by the Force. But while the former sees all life as part of the Force, the latter sees everyone as a slave to the will of the Force. So Dark Side users are those who can fight the fate the Force is, well, forcing on people. The Acolytes believe they are being directed by visions of Sith who have died. The cell on Devaron also has its hands on a Sith lightsaber.
One of their living masters is Yupe Tashu, who advised Palpatine and aids Rax. Tashu is all in on the power of the Dark Side. Tashu believes that there are masks which have soaked up the Dark Side of the Force and can give power. He might be on to something, since he gives a mask to an Acolyte who is swept away by her anger, and uses it and a lightsaber to win a battle on Devaron. That belief in masks seems exactly like something Kylo Ren and the Knights of Ren would take up and helps explain why Ren wanted Darth Vader's mask so badly.
The Birth of Ben Solo
Eventually, after the Empire loses on Jakku and Imperial ruling council member Mas Amedda crawls out of where Rax tried to imprison him, Amedda contacts the New Republic to surrender on behalf of the Empire, ending the war. On the day the "Instruments of Surrender" are signed, Ben Solo is born.
Ben Solo's birth is important enough to be the subject of a billion different rumours, told all over the galaxy.. Luke Skywalker might have been there, the stories differ. The stories also differ on whether it was a difficult or easy birth, and whether he was born with a full head of hair. One story mentions that Ben was born with a full set of teeth -- which is an omen we've given Napoleon, Ceasar, and Henry VIII. That is a lot of speculation and expectation for one kid to grow up with.
Leia also described her unborn baby's presence in the Force as "He is less a human-shaped thing and more a pulsing, living band of light. Light that sometimes dims, that sometimes is thrust through with a vein of darkness." Some not-so-subtle foreshadowing, even though Leia admits that Luke told her everyone mixes Light and Dark like that.
The New Republic
The new canon books set in the post-Return of the Jedi, pre-The Force Awakens period have all made very clear why Leia has to found the Resistance rather than fight the First Order with the New Republic. And that reason is basically that the New Republic is mired in slow-moving politics. Throughout this book, the people running against Mon Mothma make it incredibly hard to strike back at the Empire, even though they know that what's left of the Empire's fleet is hanging out above Jakku. Mon Mothma loses the first vote to authorise military action and that stalls the final battle. If Rax's plan wasn't to fulfil the Emperor's goal of burning it all down anyway, they'd have lost their chance.
Another example of the way that the New Republic is more political than practical is that everyone involved in the liberation of Kashyyyk from Aftermath: Life Debt gets a commendation and are exiled: A commendation because it was a successful action and, PR-wise, it looked good, and exile because it wasn't actually an authorised mission. As a result, people like Wedge Antilles -- Wedge fucking Antilles -- are given jobs with no actual worth.
Meanwhile, the New Republic isn't the only group to spring forth after the Empire's fall. Mon Mothma names other independent groups springing up across the galaxy in the power vacuum the Empire is leaving: The New Separatist Union (presumably a callback to the Separatists of the Clone Wars), the Confederacy of Corporate Systems, and the Sovereign Latitudes (the name pirates have given to their attempt at self-governance).
Image: The Force Awakens, Lucasfilm/Disney
The Foundation of the First Order
This book also explains why Armitage Hux (Domhnall Gleeson) has so much influence in The Force Awakens. It was hinted in the previous book, but Empire's End says for certain that Armitage's father Brendol Hux was the architect of a system that produced highly loyal, highly lethal child soldiers. Brendol, who doesn't like his son and has clearly treated him poorly, is ordered by Grand Admiral Rae Sloane to train Armitage in the methods he used. Rax also puts him in charge of the first-generation of his father's new troops. That's all the background to this exchange from The Force Awakens:
Kylo Ren: How capable are your soldiers, General?
General Hux: I won't have you question my methods.
Kylo Ren: They're obviously skilled at committing high treason. Perhaps Leader Snoke should consider using a clone army.
General Hux: My men are exceptionally trained. Programmed from birth.
Most importantly, Armitage Hux and his loyal and vicious children, his father, and a number of officers deemed "loyal" to the Emperor are charting a path through the Unknown Regions using the calculations Palpatine had ordered made. This is undoubtedly the core from which the First Order will rise.
The Return of Chewbacca's Son Lumpy
OK, maybe this isn't really plot relevant but it's very satisfying. Way back in the oft-mentioned, but impossible-to-endure 1977 Star Wars Holiday Special, we met Chewbacca's lovely wife Malla, ahis father-in-law Itchy, and his son Lumpy. Lumpy was turned into an actual character over the course of the old Expanded Universe, and his name became "Lumpawaroo," which itself became "Waroo," a definite improvement over "Lumpy."
Chewbacca and Lumpawaroo are reunited after the liberation of Kashyyyk. So there's at least a small part of the Star Wars Holiday Special that is now authentic canon. How you feel about that is entirely up to you.