Sorry I haven't responded sooner...
Both Modern and World at War are core rulebooks for the series .They each have the 42 or so page core rules you would need to play any Evolution title.
Apocalypse-Z is a supplement for either book (indeed we used WaW soley for the playtesting since Modern was not yet released). It includes new Advanced Rules, New Traits, New army lists (indeed, I challenge you to find a single zombie rhinocerous in Modern or WaW) and Scenarios...
Apocalypse-D will be similar. New Advanced Rules will not (cannot under OGL) change anything in the core sections of Modern or WaW. One such new advanced rule is:
Beastmastery: This is what allows a neanderthal with th aptly named Beastmaster trait to poke, prod, cajole, ride or otherwise badger a dinosaur into combat. There is a required number of beastmasters for each dinosaur (1/2 the size value of dino= minimum number of beastmasters). If the dino ever has fewer than the required number, there is a chance it will revert to its instinctual behavior: predatory or herd. If this happens, mayhem ensues as your T-Rex eats the rest of his beastmasters and then runs amok! Fun Fun Fun
There are also new traits (like Beastmaster). One is cavalry. This allows riders. There are also Ambush, Countercharge, and TAC Com gear to name a few. There are also heroes with heroic abilities.
If anyone wants a Demo set or rules (just a taste) go to the Exploding Goat Games Yahoo group and sign up. Members of the group can download the demo free from the files section.
We're hoping that this will also be a Flaming Cobra imprint taken on by Mongoose (as they did with the upcoming Apocalypse-Z [YAY])
Thanks for listenin'
Jay Hyland
President of Exploding Goat Games