My guess is that it's going to be like GURPS.
You have a base system. Look at 3e. Your base class framework (BAB, saves, HP, etc). Basic skills/spells, equipment.
In this book, we'll call it Tome of Sweat (thank you @
Dice4Hire ). This is your Gritty Book. It has rules on how to make the game more lethal and gritty, save & die effects, it gives you a formula on having everyone start with reduced HP, on having non-awesome powers, etc.
In this book, we'll call it Book of Awesome. It fits on top of the basic framework to effect classes, and give powers.
Then this book is Book of Combat Expertise. It's a grid-based system (because they are saying the base system will not be on a grid).
Then there's the Book of World-Emulation. This is your simulationist "how many commoners/experts/high level PCs per 1,000 population, thorough NPC creation rules" type of book. Encumbrance rules, indepth craft rules, etc.
A DM may pick up the Book of Sweat and Book of World-Emulation, that goes for his table. Another DM could pick up Book of Awesome and Combat Expertise at his table.