Another ex-GW staffer who now writes for Black Library. THE Guy Haley also ran British sci-fi magazine SFX for a while. After a promising start with books like Skarsnik, Champions of Mars and Crash, Guy has mostly gone for middling Black Library pulp. Because you love that shit. He is also one of the few writers in Black Library to write stories from the perspective of Orks (and DEYZ IS GRaTFUL FOR DA PUBLISITEE).
More recently, Guy has established himself with forays into the Blood Angels, Dark Imperium and the Horus Heresy. All of which prove several things.
He's awesome at Blood Angels. He made Dante so cool not even Mary Sue Syndrome can touch him. He gave the Blood Angels fortress monastery its badass name "Arx Angelicum". He detailed their narrow victory over Hive Fleet Leviathan following the disaster at Cryptus. He's also written a decent bit of Night Lords and Ultramarines material, the latter including the first major treatment the Primaris get in novel canon.
He does have the unfortunate tendency to shy away from actual combat, and has a hard time making massive battles feel massive. The fighting at and around Baal lacks the weight and scale that you'd expect from such a monstrously huge battle, and much of it is glossed over and shoved aside in favour of character interaction.
He has a knack for writing Primarchs, both the Loyalist and Traitor ones. He also has been scheduled for the largest number of Primarch books so far, including Perturabo, Corvus Corax and Konrad Curze. Currently he's taken up the role of turning Guilliman into an awesome (ish) protagonist with the Dark Imperium series, which means he gets to write Mortarion. He also wrote the definitive novelized version of Guilliman's confrontation with Fulgrim.
He's descriptive to a fault, and has a lot of occasion to write Nurgle, which is as disgusting as you might expect: in the Dark Imperium: Plague War, he writes several paragraphs worth of a man being turned (literally) inside out by the powers of Nurgle and it's almost as horrible as the Daemonculaba. Unsurprisingly he doesn't do a lot with Slaanesh.
Not Ian Watson.
Haley has never met an adjective or adverb he did not like. The effect can be cringe-inducingly cringeworthy as a cringeing... you catch my drift. “With a lurching flop, the clock-daemon lurched forward...” For the love of... Not every sentence has to be a flowery description or metaphor.
He's apparently one of the fastest Black Library authors.
He also looks like the love child of Roose Bolton and Martin Freeman.
He also helped write the fluff for kings of war third edition.