Catalyst, by James Luceno, is the companion novel to Rogue One, the first standalone movie in the Star Wars franchise. Lucasfilm/Disney seem to have settled on providing more background by the means of novels. It has been done for The Force Awakens and the Rebels TV show before.
Catalyst tells the story of how Galen Erso came to be involved with the Death Star project, and gives more background on Orson Krennic. It also introduces Saw Gerrera and conveys a sense of belonging to the bigger Star Wars universe via frequent appearances of Tarkin. But besides that, it is a straightforward story of Galen’s involvement and withdrawal some time before the start of the movie.
It’s not a literary masterpiece. It’s not universe-shaping like the Thrawn trilogy was, and rarely did I feel much suspense. Maybe it’s because we know what will happen due to the movie, but everything seems quite straightforward and inevitable – no surprises and sudden plot twists.
As such, I can recommend Catalyst to those who want to know more about the backstory. If you’re only interested in the bigger picture, feel free to skip it – the real action happens in the movie.