Wanted to do a roundup of the biggest hints/reveals/easter eggs that can be found in some of the books that were released after the premiere of TFA. The books I'm referring to are "Star Wars TFA The Visual Dictionary.", "Star Wars Before the Awakening", and the novelization for the movie. After all the main information I'll have a section for much smaller bits of information that I still found to be interesting. You can skip those if you want. Add other things you noticed that wasn't mentioned here!
From the novelization (I didn't actually read this one, so I'm using insights from what I gathered on the internet)
1) Snoke and Kylo knew about Vader's turn to the light in ROTJ.
2) The New Republic does not take the Resistance seriously, leading to a growing resentment within Leia.
3) When Rey uses the force to grab Luke's lightsaber, Kylo mutters "It IS you". Kylo having previous knowledge of Rey, confirmed!!
4) Leia knew Snoke was trying to turn Kylo to the dark side, but didn't tell Han because she thought she could save him without Han being involved. This is what ultimately led to Han and Leia separating.
5) Kylo doesn't feel good about killing his father. When Kylo killed his father, he was "stunned by his own action... Following through on the act ought to have made him stronger, a part of him believed. Instead, he found himself weakened."
From TFA The Visual Dictionary:
1) As was mentioned in "Aftermath" also, the New Republic signed a Galactic Concordance with the weakened Empire soon after the events of ROTJ. This treaty prevents both sides from fighting each other, and completely demilitarizes what's left of the Empire. They no longer see the remnants of the Empire as a threat, sadly they were wrong.
2) The New Republic Senate moves to different planets on a rotation-based system. At the time of TFA, the Senate was located on Hosnian Prime, which is why the Hosnian System is targeted and destroyed by the First Order.
3): The old man at the beginning of the movie, Lor San Tekka, is described as a "seasoned traveler and explorer of the more remote fringes of the galaxy... has proven his worth to the New Republic and the Resistance many times over... ready to retire after decades of exploration and adventure... old enough to have witnessed the Jedi Knights prior to the Clone Wars... uncovered much of the history of the Jedi Knights that the Galactic Empire had tried so hard to erase"
4) Finn is at least 23: "23 standard years spent training (from birth)"
5) Kylo Ren is "no Jedi, and nor is he a Sith. He is the archetype of a new generation of dark side users that have emerged to fill the void left by the Sith's demise."
6) "The supreme leader believes Ren to the ideal embodiment of the Force, a focal point of both light and dark side ability."
7) Kylo's helmet is "patterned after the battle gear of the Knights of Ren"
8) In regards to Kylo's lightsaber, "its components are modern. Its crude appearance suggests construction by an inexperienced hand" The lightsaber also uses a cracked Kyber crystal, explaining the unstable appearance of the blade. "The stressed crystal barely contains the power of the weapon, necessitating lateral plasma vents that become the crossguard quillons."
9) The book mentions that Ren is "the most gifted apprentice of Supreme Leader Snoke". So Snoke has other apprentices?
10) The book also confirms that Ren did indeed kill the Jedi Luke was training: "Ren betrayed the other Jedi students studying with Master Luke Skywalker, and is responsible for their destruction."
11) Snoke is "steeped with Dark Side knowledge"
13) The rebel helmet Rey wears is confirmed to once belong to "Captain Dosmit Raeh of Tierfon Yellow Aces" (Sorry for those who thought this was Luke's!)
14) Rey is 19 years old in TFA (So Finn is 23, Rey is 19, Poe has to be around 30 (Conceived on Endor), and we don't know how old Kylo is I think)
15) Maz Kanata has "an affinity for the Force. Though she has known Jedi, she has never walked that path herself, instead relying on her strong connection to the Force to keep her out of trouble."
From "Before the Awakening":
1) Finn is kind of a bad-ass... he is considered one of the best, if not the best, Stormtrooper cadet. He is also the leader of his four-man squad. Captain Phasma has high hopes for him.
2) Despite his combat excellence, he is isolated from the rest of his peers. Among themselves, the stormtroopers go by nicknames. Finn, while still in service, is still referred to as FN-2187 by his peers.
3) Rey salvages a pretty top-notch flight simulator from one of the ships she scavenges for a living. It has several ships to choose from that she can fly. She uses it relentlessly. This is a big contributor to her piloting skill.
4) There is a New Republic Fleet located on Mirrin Prime a several months or so prior to the movie, perhaps the New Republic is not completely gone?
5) Although the Resistance is considered smaller than the Rebellion, there is at least one Mon Calamari Cruiser in service, The Echo of Hope. Perhaps there is a semblance of a Resistance fleet?
6) Poe was a New Republic pilot but left because they were not taking the threat of the First Order seriously.
7) Poe manages to successfully steal a New Republic Senator's cruiser, proving that the Senator is a First Order spy. The ship contained First Order intelligence, such as information indicating Lor San Tekka's ownership of the missing Luke Skywalker map. How many other senators were working with the First Order?
Here are a few smaller, but interesting things I noticed as well:
1) I can't remember if this was mentioned in "Lost Stars", but Jakku once held a secret Imperial Research Base
2) D'Qar, the planet home to the Resistance Base was founded by Thane Kyrell and Kendy Idele--two big characters from "Lost Stars"
3) All ejector seats in Tie Fighters have emergency rations underneath. Obviously Finn was unaware of this, lol.
4) After the events on Jakku, Finn is "scheduled to undergo 'renewal therapy' to ensure his unquestioning loyalty to the First Order."
5) The new Resurgent-Class Star Destroyers are "almost twice the size of the Old Empire's Star Destroyers."
6) Rey crafted her Rebel Pilot doll when she was 10 years old
7) Rey built her speeder
8) Sarco Plank, the villain in "The Weapon of Jedi", is basically "retired" now and works as an arms dealer at Niima Outpost
9) Chewie: "At 234 years old, Chewbacca is still not yet approaching middle age for a Wookie"
9) The red-masked thugs from the Guavian Death Gang are illegally and Cybernetically-enhanced
10) For those who have read "Aftermath", the big bearded pilot for the Resistance is named Snap Wexley. He is "the son of a Rebel Alliance Y-Wing pilot who flew at the Battle of Endor." Sounds a lot like his mom is Norra Wexley. So is this Timmins? Why is his name Snap?
11) Jess Pava, the pilot in the prologue and epilogue of "Weapon of a Jedi" is the female Resistance pilot in the movie.
12) One of Finn's squad mates and perhaps Finn's only semblance of a friend, FN-2003, is the stormtrooper that dies and wipes a streak of blood on Finn's helmet. FN-2003's nickname among the troops was "Slip" because he constantly messes up, and Finn busts his butt time and time again to help him so he doesn't fail. Phasma encouraged/ordered Finn to stop doing so as it only weakens Slip. Ultimately it seems she was right, as Slip dies on his first real mission.

So I've been reading the novel, and found a scene that I think does a good job explaining why Ben kills Han. I can't remember if this quote is in the movie or not, but it's when Snoke was talking to Kylo. I've quoted below.
"The historians have it all wrong. It was neither poor strategy nor arrogance that brought down the Empire. You know too well what did."
Ren nodded once. "Sentiment."
"Yes. Such a simple thing. Such a foolish error of judgement. A momentary lapse in an otherwise exemplary life. Had Lord Vader not succumbed to emotion at the crucial moment-had the father killed the son-the Empire would have prevailed. And there would be no threat of Skywalker's return today."
"I am immune to the light," Ren assured him confidently. "By the grace of your training, I will not be seduced."
"Your self-belief is commendable, Kylo Ren, but do not let it blind you. No one knows the limits of his own power until it has been tested to the utmost, as yours has not been. That day may yet come. There has been an awakening in the Force. Have you felt it?"
I thought this was really interesting because it really gives some insight into why Kylo thought he had to kill Han. He wants to finish what Vader started and accomplish what he could not. He saw Vaders act as a momentary lapse of judgement, not him turning away from the dark side to the light. Not killing Luke was what destroyed the Empire. He didn't want to make the same mistake and do something that would lead to the destruction of the First Order. If he didn't kill Han, it would mean giving into the light like Vader did with Luke. I thought this was a cool exchange that really set up that moment in the movie. I don't remember it in the film, but I've only seen it once. I do remember him saying there has been an awakening, have you felt it.