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. 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 since it 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 discrete 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?
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?