I would recommend "Deliverence Lost".
It continues a storyline you have already started. Would just be a spoiler if I tell you which one. 😁
In general this book does a lot for the world building. It explains the history of the MK5 and 6 in detail, we get some interesting stuff about the creation of the Astartes in general, and with kurze against corax on istvaan, one of the best fights between primarchs. Nit for it's combat discribtions, but what it does for both as characters.
And just the tip of the iceberg. This book does everything right.
After that you should read the calth plotline. (First heretic, fear to thread, betrayer, mark of calth). Just because "Betrayer" is soooooo good. It's from ADB (author of the famous night lords trilogy, that btw get tons of important flashback into the heresy, as well, so read that for NL context in the heresy).
In that book you will learn to love Kahrn, Argel Tal and maybe even Lorgar. And you will hate Erebus afterwards so lich kore, than you already do. 😂
Also Betrayer has the most badass human in the story. Shes a warhound pilot and friend of Kahrn.
Also the short story collection books are really good.
Age of darkness, the primarchs and shadows of treachery tell a lot of important sideline stuff. Like the complete story if the war between the DA and NL. What happens to fulgrim directly after Istvaan, a additional story that follows up on "deliverance lost" and more.
The Vulcan lives and the blood angle novel on that demon world. (Can't remember the name), before you should read "Imperium secundus". If you read all form my list, you have followed every plotline that is tight up together at Imperium secundus.
The First Heretic, Fear to Tread, Betrayer Those are the next highly acclaimed trilogy and makes sense as the next step from the first five.
Anything by Dan Abnett, Graham McNeill, John French and Aaron DB
I would recommend "Deliverence Lost". It continues a storyline you have already started. Would just be a spoiler if I tell you which one. 😁 In general this book does a lot for the world building. It explains the history of the MK5 and 6 in detail, we get some interesting stuff about the creation of the Astartes in general, and with kurze against corax on istvaan, one of the best fights between primarchs. Nit for it's combat discribtions, but what it does for both as characters. And just the tip of the iceberg. This book does everything right. After that you should read the calth plotline. (First heretic, fear to thread, betrayer, mark of calth). Just because "Betrayer" is soooooo good. It's from ADB (author of the famous night lords trilogy, that btw get tons of important flashback into the heresy, as well, so read that for NL context in the heresy). In that book you will learn to love Kahrn, Argel Tal and maybe even Lorgar. And you will hate Erebus afterwards so lich kore, than you already do. 😂 Also Betrayer has the most badass human in the story. Shes a warhound pilot and friend of Kahrn. Also the short story collection books are really good. Age of darkness, the primarchs and shadows of treachery tell a lot of important sideline stuff. Like the complete story if the war between the DA and NL. What happens to fulgrim directly after Istvaan, a additional story that follows up on "deliverance lost" and more. The Vulcan lives and the blood angle novel on that demon world. (Can't remember the name), before you should read "Imperium secundus". If you read all form my list, you have followed every plotline that is tight up together at Imperium secundus.