There are currently
Five Red Books for 30K.
1.
HH: Age of Darkness Rulebook- Just the rules. It's the Warhammer 40,000 7th edition rulebook rewritten for the Horus Heresy 30K.
2.
HH: Legion Astartes: Age of Darkness Army List- The "generic" 30K Legion Army rules, including force organization, Rites of War, War Gear and Equipment, Missions, and deployment
3.
HH: Legion Astartes: Age of Darkness Legions- Legion Specific rules for all the Legions EXCEPT Thousand Sons, Space Wolves, Dark Angels, Blood Angels, and White Scars
4:
HH: Mechanicum: Taghmata Army List- Playing Ad Mech in 30K and the three factions (not Dark Mechanicum)
5:
HH: Crusade Imperialis: Army Lists- Solar Auxilia, Knights and Titans, Imperial Militia and Cults
Thousand Sons, Space Wolves, and Custodies can all be found in
HH: Book VII: Inferno. Hold Over rules for Dark Angels, Blood Angels, White Scars, and Black Shields (dispossessed Legion armies) can be found in
HH: Book
VI: Retribution.
By the end of the year, Forge World says it will release Book IX: Malevolence and its all about Blood Angels, White Scars, deamons, and some additional rules for Space Wolves and Alpha Legion, and new campaign system.
Book 10 is presumed to have Dark Angels and Dark Mechanium, and beyond that... no idea. Forge World stated in 2020 when all the Legions are done, the Red Rule books will be redone one more time. I'd presume in preparation for the next rules edition of 30K.