Right finished the book my first response. This book is a MUST for death fans in my opinion this book goes into what nagash has been planning. I have been making the point that Death only got a page in the all-gate's book but this book developers it further(Plus neferata's story from balance of power) all the Mortarch's appear in the book and we get to see what happened to mannfred after his escapade.
On mannfred and if he is independent or not
Nagash's plans and the stormcast (I feel it's pretty big) BIG SPOILERS.
Now as I said Death players who want fluff need the book
IMO. It's quite a fun read and it's nice seeing things from neferata's/nagash's perspective as well.