This is the third book in the series (despite sharing the same name as the first), and the other two Imperial Armour Apocalypse's (1st and 2nd editions) have been discontinued, so this book is effectively their full replacement. It costs £30 which, whilst more expensive than the other editions, it is on par with current hardback codexes and is bigger. The surface of the book has also changed, from a high-gloss to a smooth satin finish.
Imperial Armour Apocalypse contains 161 pages of pure 40k rules and units. For reference, the previous book, Imperial Armour Apocalypse: Second Edition, only had 128 pages. There are no sections of massive fluff, only a paragraph of so above each unit detailing its history. This fluff cutback means there is space for plenty of new and old units contained.
Most units available from Forge World are in this book. As chaos player, I am happy to see that I no longer need to carry around two massive tomes every time I want to play both a Contemptor Dreadnought and a Storm Eagle, both are in this book. The only things that I see omitted from the Chaos section are the "hell" planes. I also cannot find the Rapier guns, but I suspect they are a Horus Heresy special.
We are also treated to a number of new units, such as the "relic predators" for both good and evil space marines, Necron Tesseract Arks, small-scale Pylons and even the city of the dead, a table-wide fortification.
In terms of editing quality, Forge World has managed to solve most of the issues from previous books. I remember in IA:A 2nd ed that my army was referred to as the Thousend Sons. That has been fixed. Also, every unit now bears a mark, saying if it is an apocalypse only unit, or if you can have fun with it in your standard games. In the latter's case, it also says what slot it occupies, something that was missing from a few in the last book.
Whilst they have fixed a few issues from previous books, some still remain, and new ones have popped up. One example is the Minotaur siege tank, where it says it is equipped with a pair of heavy bolter sponsons. Anyone who hows the model know there are none. It seems that Forge World's well-known proofing errors haven't been fully resolved.
What is new about this Apocalypse book, it is it the first one I've seen in this series that has added named characters to the book. All the named characters from the three Vraks campaign books (Mamon, Zhufor, Rex etc.) have been updated and included in the book, for use in your campaigns or even normal games (and in which armies). Alongisde these characters, you also get an updated campaign set to reenact the Vraks siege.
So, to conclude, you are getting a much bigger and tightly packed book, full of new units and old ones with fresh new looks. Ignoring the odd grammatical error, this book is a fantastic purchase. And in a day and age where GW charges an arm and both legs for it's models, just think: Forge World is looking a lot more affordable isn't it?