The Warhammer 40,000 Codex books contain the complete rules for playing various armies in Warhammer 40,000. They also contain in-depth painting guides, army lists, and much more.
Space Marine Codex 6th edition allows you to use the updated Space Marine forces. Rules for new units and chapter abilities are introduced. (user summary)
AND THEY SHALL KNOW NO FEAR!
In the grim darkness of the 41st Millennium, the Imperium of Man is assailed on all sides by rebellion, alien invasion and the bloodthirsty servants of dread and terrible gods. Yet there is one force that forever stands against these threats, one force whose destiny is an eternity of battle - the Space Marines. These superhuman warriors have safeguarded Humanity against the dark and hungary galaxy for ten thousand years. They have fought the most thankless of wars against the most terrible of foes, winning impossible victories when all around them have fallen. They are the Adeptus Astartes, the Angels of Death, and they shall know no fear. (from the back cover)