I just finished reading this one. I liked it!
The book gives you a much more in-depth view of how the relationship between Galen/Mira/Jyn, and Director Krennick came to be, developed, and turned into what we saw in Rogue One. Also, it articulates the competition, and antagonism between Tarkin, and Krennick through a series of efforts directed at each other for the purpose of weakening the other's standing with the Emperor.
I would have liked to know more about the nature of the Kyber Crystals, and how their power was harnessed, and perverted to become such a fearsome weapon, but perhaps that is one of those things that they intentionally leave ambiguous, so as to let the reader's own imagination fill the gaps.
Overall, a decent investment of time if you have an interest in what preceded the events of Rogue One.