Aftermath and Lost Stars are a pair of the new books in the current Star Wars canon, and both proudly proclaim on their covers that they're part of the "Journey to Star Wars: The Force Awakens" and while that might be true, it's less true than you might hope. Neither book really ties in very much with the events of TFA.
The Lost Stars takes place mostly during the original trilogy. And at 551 pages, that's a lot of ground that we've already covered. The end of the book does, finally touch on TFA, but not before giving us cameos of just about every major imperial and rebel character from the original trilogy.
Aftermath takes place post Return of the Jedi and covers what happens pretty much immediately after the destruction of the second Death Star. It has an entirely new set of characters on a new planet, with Wedge Antilles being the only character from the original trilogy to make an appearance.
Aftermath is everything I should want in a bridge novel, expanding the universe of Star Wars, focusing on characters and events that don't tie directly to the Skywalker family. And yet it was Lost Stars that was the superior novel. I actually cared about the protagonists, and getting to see familiar events from a new perspective made them feel fresh. While I knew what was going to happen, big picture, I didn't know what would happen to the characters, or how they would react, or even if they'd survive. It felt like a real Star Wars story, with sweeping events, good vs evil, right vs wrong.
In comparison, Aftermath felt like someone's transcript from a long RPG campaign. I felt like I could grab the character templates from the West End Games rulebook, and tell you what each character used, Bounty Hunter, Kid, Rebel Pilot, Ex-imperial... It seemed like it was a fun sci-fi game, but, it lacked the sweeping grandeur, the epic space opera feel of Star Wars. Maybe because the events were primarily confined to a single planet?
One thing that Aftermath had that I did really enjoy was a series of interludes that give you glimpses of what else was going on in the universe... but because they weren't connected either to each other or the main storyline, it just didn't work, didn't flow the way it could have. And really I wanted those interludes more than I wanted the main story.
I'd recommend Lost Stars if you're looking for a good Star Wars book. Yes it's long, but it's a young adult book, and so the pages will fly by.
Aftermath might be worth reading if you need to read everything Star Wars, but only then.