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.