If you want the full experience you could read Catalyst, which essentially tells the story of the Erso's and Krennic. It basically tells the story leading up to the first scene in Rogue One. The book itself is just ok, but I read it before going to see Rogue One in the theaters so it was cool knowing the entire background leading into the movie.I watched Rogue One last night, followed by the opening scene of A New Hope.
I have adored Rogue One since its release. It's a gorgeous movie and the characters are compelling.
But...my goodness! Everything is now richer and has more depth. Jyn Erso feels less like the main character (I didn't hate that like some fans did) and Cassian feels fully formed.
One unexpected treat was that the opening crawl of A New Hope is now completely informed by Andor/Rogue One. I mean...