You can find digital versions or summaries of these materials on hobbyist platforms like or lore repositories such as of the Battle of Calth or the tabletop rules for the Imperialis Militia found in The Horus Heresy V - Tempest RULES PDF - Scribd
The "Betrayal at Calth" campaign allows players to refight historical battles with unique missions, maps, and Age of Darkness expansion rules. Horus Heresy Book Five Tempest Pdf 39
These are now officially out of print. They were massive, leather-bound collector's items. Finding a physical copy often requires scouring secondary markets like eBay, where prices can be quite high. You can find digital versions or summaries of