Please remember that these are rumors.
via a Faeit 212 "must remain anonymous source"
I was talking to about it and they said that it would be several books not one (one said 6+ another said, "yeah, 8"), the first of which begins after "A Galaxy in Flames" in terms of Time-Line, so there will not be rules for the Legions before the heresy (i.e. Loyalist Luna Wolves/Sons of Horus, Death Guard, Emperor's Children).
Each book will focus on a specific thread of the campaign toward terra, culminating in a final book which will be the battle for terra itself.
Books will contain expanded fluff for the 2-3 Legions contained within and campaign rules for playing out a particular series of battles with "benchmark" missions that are historic. Think the Badab war campaign.
It is being worked on congruently with Imperial Armour and will not delay or impact the IA series.
There will be new rules for the Legion standard, and will not use "Legion Tactics" (ala Chapter Tactics) variation based on special character, but will instead be edited in some way based on simply nominating the Legion you're using.
The current forecast for models will be upgrade packages -- specifically shoulder pads for the various legions and special characters. These special characters will all be unique and new, and not those from the Black Library series.
Primarchs will be featured and will have models, but are apocalypse only.
The first book is expected for October and will feature Luna Wolves and Dark Angels in terms of fluff and models, but no word on what campaign it will involve.