Flight of the Eisenstein is pretty good too. But... none of the HH books have ever really grabbed me. 40k has some truly good pieces of science fiction (The Infinite and The Divine (probably the best warhammer book, I don't even think that's a contentious claim), Ruin/Reign, Flesh & Steel, Assassinorum: Kingmaker), but I was always kinda underwhelmed by the 30k stuff... idk what it was. Maybe the inevitability of the setting?

Not Horus Heresy, but I've also been having fun with the Fabius Bile Omnibus by Josh Reynolds and The Night Lords Omnibus by Aaron Dembski-Bowden. In general, heretic novels, at least the ones I've read, have a higher quality on average than the loyalist ones. Xenos ones can be fun too, Infinite and Divine, Twice Dead King, Ghazghkull Thraka, and Brutal Kunnin' were all fun. The best loyalist one I've found so far was The Fall of Cadia, but even that's mostly just big good guy shoots big gun at big bad guy. Might as well read Ork books at that point, they're at least having fun. Actually, that's not quite right, The Great Work is also a good loyalist one, although I'd have preferred more Cawl and less Primaris Marines.

Yeah, I don't remember the specific books but there is a specific thing that I thought was brilliant. One book follows an Astartes that starts to suspect heresy in their ship and he escapes it, he approaches another ship and recognizes the Astartes on the other end of the radio and confides in him about his suspicion and he lets him onboard. Then in another book you follow the story of that other Astartes, who starts to suspect heresy in the fleet and when the radio call comes it confirms his suspicions.

Not the greatest thing ever, but it was a cool thing that I don't think I've seen any other book series do something similar.