Dan Abnett
Series include Gaunt's Ghosts (an amazing starter book for people getting into the 40k fluff), Eisenhorn (fantastic) and Ravenor (OMGAWESOME-ICRIEDBUKKETS).
Dan is known for fun action writing and rampant sodomy of Warhammer fluff, like Imperial Guardsmen toting around Autocannons the size of a howitzer. And Being able to fight anything the 40k Universe can throw at them including killing Tanks with tennis balls. His characters are very well developed and are distinctive and orginal; he can write a god-damn action scene better than any; the areas Dan describes sound achingly beautiful; he's a versitile writer and.... Look, he's won tons of awards for his stuff, many believe he's the best Black Library have got, he's written around 20 for them and each one is brilliant. He even made his own (awesome) Space Marine chapter known as the IRON SNAKES, that comes off looking like the Ultramarines if they were actually intelligent, adapted their battlestyle to suit each enemy, and came across as likeable individuals AND godlike beings.
His graphic novel Imperial Gothic is pure sex.
tl;dr Dan Abnett is a godly writer who expresses 'Humanity fuckyeah' in the most epic way possible.
Dan Abnett has also teamed up with another BL author Mike Lee to make the Malus Darkblade Warhammer Fantasy book series. He is also the lead writer for the up coming 40k flim Ultramarine! HOLY FUCKING SHIT!!!!!!
Dan Abnett also wrote the script for the Ultramarines movie. It's a fair assumption that he only wrote for about 45 minutes worth of the (90 minute) movie, as the actions scenes are excellent and the dialog is solid, there's a whole lot of silent walks through the desert.