The Horus Heresy is a big, expansive series. So, when I'm compiling my series-wide Dramatis Personnae, list of ships or notes on timeline, what exactly am I including in this?
The following are factored in:
The full numbered series, Books 1-54, including the anthologies.
The Primarch series (18 books + the Heirs to the Emperor anthology)
The Siege of Terra series (10 books + 3 novellas + 1 anthology)
All the stories not collected into the three main series above (see the "Release list" page, but currently sitting at 4 novellas and 34 short stories)
The character novels (currently Valdor, Luther, Sigismund, Eidolon), Cthonia's Reckoning, and the date details given in the Horus Heresy Journal.
All the Forge World "black books", Books I to IX.
All published works for the 2nd edition of the Horus Heresy tabletop game (currently the rulebook, Libers Astartes, Hereticus, Imperium & Mechanicum, The Battle for Beta-Garmon, The Siege of Cthonia, The Martian Civil War & Exemplary Battles Vol. 1), including digitally released "Exemplary Battles"
All published books for the latest incarnation of Adeptus Titanicus (currently the rulebook, Loyalist & Traitor Legios + 5 campaign supplement books), including digitally released "Exemplary Battles"
All published books for Legions Imperialis (currently the rulebook, The Great Slaughter, The Devastation of Tallarn and The Rise of the Dark Mechanicum) and the Horus Heresy Aeronautica Imperialis rulebook.
The timeline details in White Dwarf #478
To be added soon:
Horus Heresy: Aeronautica Imperialis rulebook
Horus' Primarchs novel