Non-Limited-edition version also available to pre-order.
From the announcement:
Only 2000 individually numbered copies worldwide and exclusive to games-workshop.com. This is a fantastic opportunity to add the Limited Edition of the very first Codex: Imperial Knights to your collection.
This Limited Edition Codex contains all the written content of the standard codex and a separate art wallet which holds 7 Knight art prints. The codex and wallet are presented together in a black slipcase - which has a luxurious soft-touch finish.