I bought the book on thursday, finished it saturday morning. It is a short book, but I still thought it gave some good insight into the back story behind the demons of the 5th circle.
However, seeing that the Demonwar was supposed to be the fourth of the 5 riftwars, it wasn't really much of a war. A couple of skirmishes really.
The series did just feel like a lead in into the final of the 5 Riftwars, which I have heard is either going to be the Godswar or the Chaoswar.
My guess it will involve a war against the Dread (from the void).