There are now pics of the codex cover out and about, along with a description of what the Imperial Knights Companion is. Its basically a very in depth background and heraldry about the Knight Houses.
The cover at first glance looks to be another good piece of artwork on par with previous codex covers. Here is a link to check it out yourself. The image is a White Dwarf leak from the community that has apparently received their White Dwarf already.
via DietOfLiquor on Bols Lounge
The Imperial Knights Companion is a spectacular, large-format tome that delves deep into the orgins and history of the Imperial Knights, the culture of their noble houses and their complex relationships with the Imperium and the Adeptus Mechanicus.