This is the prequel story to the movie, Rogue One. It reveals the relationship between the villain, Krennic and his buddy ol' pal Galen. Galen is the mastermind behind the weapon on the Death Star but this story reveals that his original plan was for it to be used as a benevolent energy source.
Now we don't know the workings behind the scenes on how this project was made but it felt like Disney handed Luceno the idea but gave him very specific parameters in which to operate.
To put it simply, nothing really happens in this book.
You read about Galen and his wife and the birth of Jyn (main character in Rogue One). You read about the circumstances that led Galen to creating the weapon. And you find a quick snippet of another character in order to lead into the movie.
It honestly read a lot like Tarken. Very little action. Spending some time in the character's headspace but not really going anywhere.
To be sure, you won't miss out on anything if you don't read this book before watching the movie.
I haven't been too thrilled with the latest batch of "canon" literature from the Disney folks about Star Wars outside of Claudia Gray's Lost Stars. This one didn't do anything for my enjoyment of Star Wars.
Get if from the library in order to read it. You probably won't be rereading it.