Oh gee, I'm reading Aftermath right now. While I like the references and subtle plot hints for SW VII, it's really a very poorly written book. The writing style is awful. It literally goes like this:
Then: a noise.
And then: a flash of light.
I mean, that's how it is written. Literally. It's really quite painful to read at times and I'm having trouble focusing on the book. There's also loads of characters I can barely tell apart. I can't believe the editor allowed this to be published. It was such an anticipated book and the references and the plot has some nice points, especially when referencing the bigger picture of it all. But overall it's just not very good.
I am finishing it because I want to know as much as possible about the new post-episode VI setting. If it hadn't had that I would have returned it for sure.
I don't know anything about the other books. I am interested in that YA book Lost Stars and the reviews seem to be a lot better for that one over at Goodreads, so I'll give that one a shot later on. I think Lost Stars and Aftermath aren't related at all, so you can read them in whatever order you prefer. I don't know about the three adventure books