Read all about the knights of the Empire in the Knightly Orders Guide.
According to the leaked photos released all over the internet this week some of the Fine Cast models and new griffon kit will be going on sale April 7th. I think it would be safe to assume that the Empire book will be released to coincide with this.Ref 1: http://www.thefieldsofblood.com/2012/03/new-empire-range-confirmed.htmlRef 2: http://tronhammer.blogspot.ca/2012/03/empire-pics.html
Oh look, nobody knew the exact date. If you go back and read you won't see official anywhere but my post. Yes, a few people hit the nail on the head but nobody bothered to mention that it was actually posted on GW's website. So now there is no speculation about it anymore.