It's a great prelude to what goes on in Reaper of Souls and gives a lot of insight into the Angels of the High Heavens. We also get to see Zayl again, not to mention Jacob and Shanar, so that's a lot of bang for your buck right here.
The problems though start right with the cover... I really appreciate the 3d print effect on it but the paper is so flimsy I'm afraid it won't hold up through the years.
The story has it's issues as well. I was kind of expecting it to cover why Tyrael didn't fight Diablo alongside the Nephalem. Even his follower tried to go with him but Tyrael just stood there, yeah, you go do your thing. I was always expecting that after all the insight into Tyrael's actions we'd at least get an in-universe explanation for this. IRL we know it's due to gameplay reasons but I'd like to see some official fluff that would explain this...
It also leaves you with a hunger for more and not in a good way. +/- 300 pages and you feel like you could condense everything into 100. Same could be said about The Order, also by Nate Kenyon.
Still, I liked the story and what it does for the lore so it's a 4/5 for me.