Finished it today while I was stuck at work.
Overall, it's solid. The only thing I've read of Shirley's is the Rapture tie-in novel, and I see a lot of similarities--it's sort of a "history from the little guy" bit of storytelling, especially the end, where you're essentially seeing slices of Halo 2 and 3 from a different perspective. That said, it's not really a novel so much as a collection of story threads--short vignettes strung together, where literally half the characters in the book become irrelevant for the second half (only one character bridges the periods) There's not even a real protagonist. Some of the locations start sounding a lot like Space Rapture by the end (everything's going to hell!). The writing sometimes got on my nerves (frankly, it often feels a bit clunky, because Shirley seems to have these "this is a plot point and so I will have my characters dump some lore on you" problems judging by the two books.) It probably would have been stronger as two separate novels with connections between them compared to what we got.
That said... when this novel was announced I wasn't sure what there was left to add to the founding of the Covenant beyond what Contact Harvest gave us, and I was pleasantly surprised.
Those who were interested in the Prophets will find a to like here, and the book continues the recent trend under 343's direction of diversifying the Sangheili's culture into something realized and (as annoying as it is to use in these contexts) human. I wish it was stronger as a standalone novel, but as a Halo book lore guys will find a lot to enjoy, and it's a breezy read (only took me around sixish hours on an iPhone, YMMV.)