I haven't seen any scans yet but from the Q+A i have seen from people who have the book (only one I have found that has actually taken a photo of the book with himself and a dated sign in the photo) have said that on top of your normal force org you get:
1 HQ and 1-2Troops mandatory
0-1 elite, Fast, and Heavy as a detachment, HQ cannot be a named character in the detachment.
I have been deliberately trying to break the system and so far it actually seems rather balanced. Things deflate quickly when you start borrowing points from your main force. Granted I have spent much more time just playing around with CSM/Daemons which IMO is actually one of the more powerful combos despite face value of the allies rules. Oh yeah, and anything with Space Wolves. IG is not as spectacular an ally as it appears IMO.