I hope it shows some in depth versus development stuff and not just xv.
The book was announced as a preorder bonus in France some time ago and it sounds to me like it will focus only on the FFXV Universe.
Honestly I doubt there will be much new in there we don't know already.
They annouced it with this video message from Tabata:
But I'm a bit surprised that it will have an english standalone release, always thought it was going to be exclusive to this one french store.