The first three books in the Heresy series focused on Horus and his immediate retinue. The last few books have focused on individual legions seemingly on the periphery of the main event. In some ways this is interesting, but spinning out the story is becoming rather expensive. Thus, until I reached the end of the latest book I might have rated it less highly, Abnett or not, simply because it didn't seem to be particularly relevant.
Then I finished the book.
I think most Black Library readers appreciate that Dan Abnett has a particular flare for characterisation and description. This is used fully in Legion where he concentrates on the efforts of the Imperial Army as it assaults a technologically inferior world. Some of the flavour comes from the way he combines the south-west Asian ancestry of the soldiers with the military structures of a regiment that survived the pre-Imperium unification wars on Earth, which is a story in itself. This story tells of the troubles faced by a foe that uses the powers of Chaos to thwart the military might of the 670th Expeditionary Fleet and how the Alpha Legion is engaged to defeat them. A third party takes the form of a mysterious agent sent to contact the Primarch of the Legion against his will.
The tale is carefully plotted, as one would expect, but it is not until the end that two twists reveal themselves; one of them being quite intriguing, one of them quite tragic. Although the story ends at this point, a lot of the post-heresy history of this legion begins to make more sense and entirely justifies why this book is included in the series.
Of course, the ending leaves more questions than answers - what's a secret and what's a lie? - but that's entirely appropriate for this subject.