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 (yet) collected into the three main series above
see the "Release list" page, but currently sitting at 4 novellas/short novels and 34 short stories
Additional books
The Character novels (Valdor, Luther, Sigismund, Eidolon, Layak)
Cthonia's Reckoning
Dropsite Massacre
The Forge World "black books" I to IX from the 1st edition of the Horus Heresy tabletop game
All published works for the 2nd edition of the Horus Heresy tabletop game
Core rulebook, Liber Astartes, Liber Hereticus, Liber Imperium & Liber Mechanicum
The Battle for Beta-Garmon, The Siege of Cthonia, The Martian Civil War & Exemplary Battles Vol. 1
All published works for the 3rd edition of the Horus Heresy tabletop game
Core rulebook, Liber Astartes, Liber Hereticus, Liber Mechanicum, Liber Questoris, Liber Custodes, Liber Auxilia
Journal Tactica: Skitarii - The Steel Hand of Mars, Journal Tactica: The Battle of Tallarn - Part One, Journal Tactica: Mailed Fist - Legiones Astartes Super-Heavy Tanks, Journal Tactica: The Forges of Saturn, Journal Tactica: The Isstvan V Dropsite Massacre - Part One
All published books for the latest incarnation of Adeptus Titanicus
Core rulebook, Loyalist Legios, Traitor Legios
Titandeath, Doom of Molech, Shadow and Iron, The Defence of Ryza, Crucible of Retribution
Digitally released "Exemplary Battles"
All published books for Legions Imperialis
Core rulebook, Liber Strategia, Journal Strategia: The Ruin of the Salamanders, The Great Slaughter, The Devastation of Tallarn, The Rise of the Dark Mechanicum
The rulebook for Horus Heresy: Aeronautica Imperialis
Extra supplementary but canon materials
The timeline details in White Dwarf #478
The date details given in the Horus Heresy Journal
The published Forge World timeline