Warhammer 40k Rules & Codexes
All you need to play a game of Warhammer 40k is the Core Rule book and the Codex for your chosen faction. The Core Rule book contains all of the rules of how to play the game, which contains lots of visual examples, covering different situations.
Your Codex contains lots of background about your chosen faction, with lots of information about them, any sub factions, historic events and any other interesting facts. In addition to this it also contains profiles for all of the different units you can field. As well as Warlord Traits that you can assign your leader, Relics that you can equip them with and stratagems.