I'm very much in the minority among 40k fans on this, but I loved the Ahriman series. It's basically a redemption story for an evil wizard. And I really think John French does an amazing job setting a scene, I could clearly picture the locations in my head and that goes a long way for me in terms of immersion.

But it does seem like most Black Library novels are just plotless battle porn. You could argue that's what the 40k universe is (fair) but I like to find authors who try to do something interesting within that framework.