The First book was a 2-in-1 with what you need to get started and the second was akin to old “Monster Manuals” with templates to “Bring Anything”. The third book brought a “scope” of the original universe that interwove with the second and added consideration of the metric system. However, there is more than meets the eye than just “more content”…
The 2-in-1 for Players and Storytellers 
The “Bestiary” 
The Universe-Spanning Expansion
Because of the author is a one-man-army of sorts for the writing, Eon Legacy took shape in “3 Essential Parts”. The Sourcebook gave Players and Storytellers what they needed. Then The Creature Catalog provides dozens of examples and an easy-to-use template to bring nearly anything into the game. Third of all The Armory gives everyone more to play with.
Hyper Rangers for fighting Daemons, “Giant Monsters” and Kaiju. 
Power Fighters for being the living wrecking-balls that they are.
Because of the shear amount of content in The Sourcebook some will think they don’t need more. The most common statement is that it’s “what you need to get started”. However, this can’t be farther from the truth. Without The Creature Catalog and The Armory a massive part of the universe, mechanics, lore, and greater scheme of things will be missing. The author said it best:
You’ll find yourself having a play-group or campaign ready to go with all sorts of Storyteller original content and get hung-up on something. While I included the guidelines to branch from solutions in the first book the easiest way out can in most cases be found by skimming through what’s been written. I’ve had some folks bring up “What if I want to bring Monster X or Y into the scene?” which is why I made the Bestiary and all the templates that come with it. Due to the desire for “More Gear” and “More Modern Combatants” that fit more modern “Action Fiction” both Western and Eastern the best solution was the Armory.
R.A.G. Rankin