Legion

This book by Dan Abnett is the seventh Horus Heresy novel, and the strangest yet by a mile. The Adeptus Astartes are barely in it! For the majority of the narrative we're following regular human soldiers through a largely uneventful* conflict where despite the overall lack of significant threat there is a significant aura of uncertainty due to the secrecy and shadiness of the Alpha Legion, the Astartes supporting them in this arena - secrecy and shadiness only compounded by the Alpha Legion's primarch Alpharius never quite seeming to be exactly the same person twice, though nobody can ever quite exactly tell as all of the Alpha Legion look so similar. The chief secondary plotline follows a mysterious immortal human called John Grammaticus, who is on a mission all his own to manipulate the Alpha Legion into contacting and collaborating with The Cabal, an inter-species collective of concerned parties working for the future cohesion of galactic order. Without giving away too much about what Alpharius and company make of the Cabal, or Grammaticus's role in things, there is a serious bombshell in here about how an Astartes Legion may choose to throw their weight behind the forces of heresy not out of disloyalty to the Imperium but out of sheer, cold, calculated pragmatism for the greater good. This is a disarmingly gripping instalment in the series - no major epic battles, but a deeper, sharper war over trust and truth.

* So much so that there is a solid six-page passage devoted to describing a weird little game that the troops play amongst themselves wherein they have to find a rock head only just smaller than the next biggest rock head someone else has. I found this bit thoroughly entertaining.