A lot of people that play Warhammer 40k play Space Marines. They’re the iconic army of not just the game but of the entire universe. So it only seems appropriate. Well, with all those players, people are going to want to differentiate themselves from one-another, and the various Codices that exist help with that. There’s a couple new ones available to order from GW’s webshop.
The books on order are the Black Legion, Crimson Slaughter, and Angels of Death. Only one of those books isn’t referring to a Chaos faction. I’ll let you decide which (it really is grim and dark in the future, isn’t it?).
Anyway, along with the books, you can also get a Legion Astartes Psychic Powers Deck. There’s also a bunch of various army bundles available to order.