I think you'll miss nothing if you're not owner of that book, but even if nothing in it's revolutionary new it is one of the best written supplement-pieces created by 'foreign-writers'.
There's a list of the main Nepharites of Hell with different levels of power and Might. Quite a few but for my taste there's just only information about those creatures and some purgatories but nothing really new.
The Examples (for the summonings of the nepharites) given are quite nice but they're not genious.
Some Artefacts are described in Detail but they are just copies of various film-scenes or 'stolen' from various books.
The only new thing is a planned conspiracy by some of the Nepharites agains their masters... But this is for my taste a strong humanized sight for those cratures.
New Magic Rules are given (special death Magic for Pugatories)