"I’ve been a fan of Matt’s work since I first read it, and I think his Enforcer trilogy featuring the Adeptus Arbites Shira Lucina Calpurnia is entirely made out of epic win with a splash of awesome sauce."
- – Dan Abnett. (Really.)
A seriously underrated author for Black Library, Matthew Farrer happens to be one of the few authors who does not live in the UK - in this case, he is from the land down under. Farrer's mostly relegated to more obscure stuff and hasn't done a novel length work for them for a while; his work includes bunch of short stories that wind up in anthologies, including some about Eldar, the Iron Hands, the Sabbat Worlds Crusade (which earned him the above accolade from Abnett) as well as some short Warhammer Fantasy works.
Most notably, perhpas, he did the Shira Calpurnia trilogy, among the few works featuring members of the Adeptus Arbites as the main characters, and are pretty good reads at that. The first, Crossfire, is probably the best of the bunch: a detective procedural/thriller against the backdrop of the grim darkness of the 41st millenium. Throughout Farrer proves himself both familiar and comfortable with 40k lore yet willing to expand upon it to create an interesting story but without tampering too much with the universe—basically, what we want from a Black Library author. The last of the trilogy, Blind, also provides a rare and interesting glimpse into the details of the lives of Imperial astropaths.
He also wrote what is perhaps one of the best Necromunda novellas, called Junktion, which could basically be considered a standalone science-fantasy work with little connection to what we see as 40k at large (things not mentioned inclued: the Emprah, his Spess Mehreens, any sort of Xenos, the Inquisition, and Chaos.) This actually is more or less a fair fit for a novel that's about dramatic but non-world-shaking events in the lives of people who's whole universe is essentially forgotten in the Underhive.
Another short story, The Memory of Flesh, also takes up some unusual protagonists: it's about some post-Heresy Iron Hands fighting some obscure xenos who can affect the nervous system of their enemies (which becomes important.) It is perhaps the most potent portrayals of just how grimdark the whole concept of Servitors actually is, and that's when they're being used by the putative good guys. It also provides some all-too-rare general insight into the Gestalt psychology (psychopathology?) of that most-underdeveloped Loyalist chapter. Consistent with lore, but not particularly flatteringly. Read it.
Perhaps most essentially to the 40k universe, though, Farrer contributed greatly to our perception of Angron through the story After De'shea, basically defining the at-least-slightly-sympathetic modern perspective on the character - a man with noble traits but seriously fucked over by unending grim darkness, ranging from the Butcher's Nails to the seeming utter lack of pathos from the Emperor.
He is not Steve Lyons.