Rebel Rising

In a way, it's kind of like Rogue One got the most direct EU canon connections out of all of the movies so far, between this and Catalyst, Guardians of the Whills and the Rebels series (and more to come). Though (ROGUE ONE SPOILER) it makes sense, as we won't see anything of these characters post-Rogue One, and they won't contradict any of the events in the sequel trilogy or beyond.

Rebel Rising is one of the two YA novels that came out last May, centering on a few of the Rogue One characters. This one centers on Jyn, but in a way also tells the story and downfall (in a way) of Saw Gerrera (whom we'll see more of in Rebels' fourth season).
I thought it was great, and interesting. Though there were a few scenes I could have done without. (You see why it gets the YA tag, but then in other scenes this does tend to get a bit more adult). It fits in perfectly well as a prequel to Rogue One, and makes the character of Jyn a bit more emotionally rewarding (if that makes sense?) than she was in the movie. A lot of her backstory in the movie is told or implied, but having her actually go through these things and how she feels about these or just her general thought process helps a lot in that regard.

When I finished seeing Rogue One, my main thought was probably more like “I want more of these characters”, and I think this is all we can possibly get out of Jyn Erso as a character. Which is perfect, as you're not left hungry for more (BOOK SPOILER) as the book ends where the movie begins for Jyn. It feels very complete.