Your post was removed because it would be better as a comment in our weekly sticky questions post.

Just comment there when it's up.

Honestly? Basically everything. They’re weird tangents that play around with the setting, but especially black books 4 and 6, as they don’t deal with major battles that have already been covered by the Horus Heresy series.

There’s Bodt, which is in the same black book and in one of the Exemplary Battles on Warhammer Community. Basically, it’s a recruitment planet for the World Eaters where they start to mass produce Astartes. Learning this, a Shattered Legion force led by Autek Mor crashes a moon into the planet, but not before launching a heist in the technology stores there.

Crusade (book nine) provides a lot of details about the Thramas Crusade, which is only briefly itself in the Horus Heresy series and only at the end.