So, I just read Talon of horus, and I was pleasantly surprised. This isn't to say that I had low expectations for the book, so much as it exceeded my expectations for a book that was essentially a bridge between the heresy and the end of the 41millenium.
The story is well told, and resonates for me, as the theme of a military brotherhood is one I relate to. As with the horus heresy novels, there isn't any truly new material, as it is a retelling and elaboration of the raid to destroy the clones of Horus. Where it shines is in the character development. Adb did a great job portraying difficult characters like rubricae, Abaddon, and a world eater as relatable, and having human emotion, motivations, and depth.
On top f portraying chaos marines as more than cartoon villains, he showed the warp as something reflective of it's denizens, that the mutations are less a manifestation of random stigmata, but a reflection of who the mutated is at their core.
While the talon of horus doesn't really cover new material, so muchvas elaborate and clarify established canon, it offers beautiful insight into the underlying nature of the black legion, why Abaddon hasn't ascended to daemonhood, how cult marines of opposing gods can work together without significant strife, and even how chaos marines really feel about cultists( you might be surprised)