Mechanicum

This book by Graham McNeill is the ninth book in the Horus Heresy series, which I am doggedly working my way through, and oh boy this one is a doozy. Here we have a completely new angle on the events unfolding across the nascent Imperium, as we follow developments among the technology-deifying civilisation on Mars once the ripples from Horus's heresy make themselves felt across the stars. So yeh - other than a very brief glimpse of Rogal Dorn there are no primarchs in this one (we do get a fairly decent lore-dump about the doings and sayings of the Emperor himself though - and a fleeting appearance in a historical prologue), and no Adeptus Astartes at all; instead we have a diverse cast of tech-priests, Titan Legions, officials high and low, corrupt and loyal. The main character though is an ordinary human - the first, and if you ask me realistically probably last, female protagonist in the series so far - one Dalia Cynthera, a scribe with an abnormally good memory and a knack for being able to figure out how things work. Dalia is summoned to Mars by Adept Koriel Zeth, who has some hardcore secret experiments underway while simultaneously navigating tentatively through the ongoing religious schism between the Mechanicum and the Imperium. Of course, once the heresy takes root on Mars everything falls apart very quickly, and long-forgotten esoterica, blasphemous machineries and good old turf wars all play their part in driving humanity's oldest forge-world to the brink of absolute promise and utter doom. I'll be honest, this was probably my favourite one of the whole series so far - I kind of just love the Mechanicum, they're so weirdly not-quite-human but with such distinctive foibles; also Dalia was a much more compelling protagonist than the stoic, distant Space Marines who have stood at the gravitational centre of most of the books prior to this one.