Sandy Mitchell
Is not Christian Dunn. He is Alex Stewart, however, who wrote those Dark Heresy books you nerd rage about. Sandy is not a girl's name. Alex is. He writes the Ciaphas Cain novels.
He is part of the holy trinity of good Black Library authors with Dan Abnett and Graham McNeill(MIGHTY IS HE!! ) .
Style
While Abnett and McNeill are part of the trinity by virtue of writing awesome, by contrast Mitchell is part of the trinity by virtue of being fun. Remember that, "fun", the whole reason you got into the wargaming hobby in the first place? Yeah, yeah, fun is not very Grimdark, but you need a little fun to enjoy the Grimdark, because if there was none to be had it would be too depressing to play. Humor is, after all, a natural coping mechanism to tragedy and horror, and Mitchell plays that to a hilt. He is not afraid to take the piss out of the setting on occasion, and throws in a few referential jokes and plays on language for amusement.
He is fond of quirky characters who challenge their stereotype in the setting, like a cowardly commissar, a perky Inquisitor, a guardsman who prefers a chain-axe, a disgustingly filthy guardsman secretary, a Sister Superior who has a discreet sexual relationship, and a Chaos Lord who leads through speeches and has a Hitler mustache. Where else are you going to find gold like that in 40K canon?
Another of Sandy's writing features is that he actually manages to describe the worlds of the Imperium as places which actually don't suck so hard to live in, without making them sound like marysuetopias; through the eyes of his character Cain, you find that many of them are not that different from today's Earth, and people there can actually be happy as long as shit isn't hitting the fan.
He seems to have a thing about the Necrons being the most deadly dangerous damn things in all of creation. Out of all the enemies that Cain has faced, he is particularly terrified of the Necrons, to the point of almost having panic attacks at the mere memory of them. This is because Cain can fight enemies like Orks, Chaos, and even Tyranids on a fairly consistent basis, but when the ‘Crons show up there always seems to be virtually nothing that can be done about them short of overkill on a near-planetary scale.
He is also known to troll his fanbase. You would think a book named The Greater Good with a Tau and a Commissar fighting Tyranids back-to-back together on the cover would be an awesome team-up between the Imperium of Man and the Tau Empire, right? Wrong, both parties declare a ceasefire and go to defend their own domains. Problem, fanboys?
He doesn't seem to work at the Black Library anymore, not having produced anything for three years to everyone's great sadness (Actually not true anymore, it looks like he's working on a new Cain book right now). We'd all like to see how Ciaphas Cain reacts to Guilliman getting back up. (He won't. Cain is dead by that point, even though he is still registered as on active duty. Would have made for an awesome meeting between the two, though.)