Index Astartes is a feature that used to run in the White Dwarf magazine (back when it was still good). The series was presented as entries from in-universe text known (unsurprisingly) as the Index Astartes, a tome detailing the histories and characters of the Space Marines. Each article would focus on some aspect of the Space Marines -- a Space Marine Chapter (initially the eighteen known First Founding legions, and eventually other chapters that were prominent in the setting, like some of the Second Founding successors and the Blood Ravens), one of the vehicles they used (e.g. the Rhino Transport, the Predator Tank), a particular specialized role (e.g. Terminators, Techmarines), and so on. Some of these articles were collected and printed in four bound books. Similar series of articles were made focusing on xenos, entitled Index Xenos, and on the forces of Chaos, entitled Index Malleus.
With their entry into the digital age in 2013, Games Workshop has restarted the Index Astartes series (as well as creating similar series on other topics, like Munitorum to focus on weapons and Index Chaotica to focus on Chaos), selling them as individual e-books. For $5 each (though the Munitorum books are "only" $2 apiece).
Some fans who are serious about their own chapters make Index Astartes articles about them. They tend to follow this format (following the original Index Astartes articles):
- Combat Doctrine
In 2019, GW brought back Index Astartes on White Dwarf. This time it focuses mainly on the background of Chapters that have no rules in the 8th Edition Codex and actually gives them their own abilities, Warlord traits, Relics and Stratagems. It's also going to alternate with an Age of Sigmar analogue called The Tome Celestial, doing similar things for AoS armies (picking a subfaction of a single army - Stormcast Stormhosts, Fyreslayers Lodges, etc. - and giving it a bunch of exclusive warscrolls and sometimes a new named character that is just a buffed version of one of the army's generic heroes with a new command ability), Index Imperialis, for other Imperium armies, and the return of Index Xenos.