This is a toughie because it looks like you've already got some exposure to the lore. What I mention here won't necessarily be what I'd recommend to someone looking for an entry point into the whole shebang.
Imperial Guard (This category is secretly about commissars):
Necropolis - The first good Gaunt's Ghosts novel. Don't worry, it recaps everything you need to know about the main characters, and is a good introduction to the Sabbat Crusade, Abnett's little corner of the lore. And, of course, Abnett is the master of of writing humies, especially soldiers.
Honourbound - Not a series yet if you don't count the shorts, but one of the best Guard novels ever written. The regiment is interesting, but contrast Ciaphas Cain or Ibram Gaunt, Severina Raine is a more typical ruthless Commissar
Helsreach - A fairly straightforward series of events set during the Third War for Armageddon, ADB elevates the material by digging deep into what makes a Black Templar tick, and the cast is excellent
Spear of the Emperor - Especially if you'd like to get into the m42 material. The tone is pitch-perfect and it covers a lot of the detail of the new codices with finesse rather than hammering Primaris down your throat
Eisenhorn - the best jumping off point for brand-new readers, and not a bad start for people familiar with the lore already. He plays a bit loose with the lore, but it all serves the story and is often quite interesting
The Carrion Throne - If you've ever been curious about what it's like on Terra, this is the book for you. Lots of intrigue, and some of the best atmosphere in Black Library
Lord of the Night - The Night Lords book. Outlines the conflicts many traitor marines feel, offers glimpses into the Heresy, and has very little in the way of bolter porn
The Talon of Horus - Ever wonder how Abaddon rose to power? Start here, and stay for all the tidbits about life in the Eye of Terror. ADB is to marines what Abnett is to Guardsmen
The Horus Heresy
Just start at the beginning, honestly. If you've read the codices the dramatic irony shouldn't go over your head. That said, if you know how things generally play out, you can also start with something like Scars, or The First Heretic, and follow those plotlines. It would certainly be a more consistent experience.
Things like more xenos-centric books, or works about the other arms of the Imperium, are better to start on once you get a feel for Black Library.
Don't read Storm of Iron, it is bad, don't believe the memes.
Edited by Roomsky, 24 August 2019 - 08:59 PM.