Dan Abnett
Series include Gaunt's Ghosts (an amazing starter book for people getting into the 40k fluff*BLAM* one of, if not THE best IG series, full of feels and brutality), Eisenhorn (fantastic) and Ravenor (OMGAWESOME-ICRIEDBUKKETS). Also, he saved Ollanius Pius from Squat territory. He. Fucking. Saved. Ollanius. Motherfucking. Pius. He then went on to save the Squats, too, and redeemed the Ultramarines and Guilliman from Mary Sue status. Well, they're still Space Marines and a Primarch but at least they are no longer Mary Sues among Mary Sues. He also co-wrote Guardians of the Galaxy. Yeah, THAT Guardians of the Galaxy.
TL;DR: Dan Abnett is a godly writer who expresses Humanity Fuck Yeah! in the most epic way possible. Perhaps his only weakness is his marked preference for human vs human works. The number of his works that feature Xenos prominently can be counted with your hands. For those who are tired of Chaos, this can be a bad thing.
Dan is known for fun action writing and for occasional throwing off Warhammer fluff in favor of the rule of Awesome, like Imperial Guardsmen weapon teams 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. Unlike a certain molester of pen and paper, this is not done flippantly, but mostly for the betterment of the work itself. He does occasionally do some stupid shit though, like letting Vulkan grow back a second fucking head in the space of about half a second... Then again Vulkan is a Primarch and indicated to be a Perpetual.
Suffice to say, his characters are very well developed, and are both distinctive and original. He is a versatile writer that can write a god-damn action scene better than any; the areas Dan describes sound achingly beautiful. 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 books for them and each one is brilliant. His book Titanicus is currently the only book wherein the Adeptus Mechanicus are shown in a competent light, interacting with normal humans realistically and aren't all evil kleptomaniacs. 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 depending on the enemy (they do have a disdain for tanks though), and came across as likeable individuals AND godlike beings. His graphic novels include "Imperial Gothic", "Damnation Crusade", "Lone Wolves" and "Condemned By Fire".
Dan Abnett has also teamed up with another BL author Mike Lee to make the Malus Darkblade Warhammer Fantasy book series. Dan Abnett also wrote a book detailing an adventure of Matt Smiths incarnation of the Doctor for the Doctor Who 50th Anniversary collection, which is one of 11 books, 1 for each Doctor. Dan Abnett also wrote the script for the Ultramarines movie. It's a fair assumption that he only wrote for about 45 minutes work of the (90 minute) movie, as the action-scenes and dialogues are passable, but there's a whole lot of silent walks through the desert.
He, along with Sandy Mitchell and Graham McNeil make up the holy trinity of awesome 40k writers. Obviously, Dan takes the role of God. (WHAT DOES THAT MAKE ADB THEN, VIRGIN MARY? They're the Chaos Gods of awesome 40k writers, obviously, with Abnett as Tzeentch, Mitchell as Khorne, McNeil as Nurgle and ADB as Slaanesh. Obviously. Given ADB's love for World Eaters, this makes the comparison hilarious.)
He also writes some pretty cool comics with his probably-best ex friend Andy Lanning. The two are known in comic reading circles as "D'n'A". Surprisingly, this list includes IDW's ongoing Transformers series. So maybe, just maybe there is a teeny tiny reason to fear a Transformers-Warhammer 40k crossover somewhere down the line.
It should be noted that his cosmic Marvel comics with probably-best friend Andy Lanning are not at all shit, and are in fact some of the greatest modern Marvel comics. The Thanos Imperative stands out amongst them, and is worth a read even if you have no familiarity with Cosmic Marvel or the Marvel Universe at all. D'n'A also wrote the reboot of Guardians of the Galaxy, which now has a movie. A movie that is pretty much loved universally, and considered one of, if not THE best in the MCU (which you may have heard of). He also helped write the script for the new Aliens Isolation game just so you know.
He also helped write a Judge Dredd comic called Insurrection, not sure on quality but the villains look like Necrons on the cover. It's military sci-fi with power armored Mega City 1 Spess Mehreens going Exterminatus-lite on what essentially amount to the heretics of that universe which he wrote based on the 2000AD people asking for a story "like the stuff you do for the other guys".
Not to Say He's Perfect
Abnett has written some fantastic sci-fi, true, but recently there have been a lot of grumblings on /tg/ about the changes he's made to the Horus Heresy fluff. A lot of fa/tg/uys feel that additions such as the Perpetuals, the Cabal, and Enuncia were shoehorned into 40k fluff and don't belong. The change of Ollanius Pius from a badass regular guardsman to an immortal Perpetual has received a mixed response, and a lot of people felt the addition of the Alpha Legion's subplot with the Cabal (You guys gotta' help Chaos to destroy Chaos or else the Emperor will become a god and by the way humanity has to go extinct too sorry guys) was retarded and poorly thought out. Also, Unremembered Empire is generally agreed to be a shit burrito, but much of that was the result of Abnett trying to tie together a long line of shit produced over the course of the Horus Heresy series.
His Gaunt's Ghosts series is another victim of obscene reasoning, be it their ability to wear hobbit cloaks that make them so sneaky they cannot be found with Auspex, the Chaos Space Marines falling to blow darts(except that they were also poisoned, and only distracted one, while the second took dozens to the face to die), or simply wearing more plot armour than Batman. His love for the colour ochre was worrying, pre-dating Tau fashionistas. Perhaps his worst sin is obsessing with pretty mary sues whose gorgeous looks prevent execution, despite the chapter prior saying they had just died (I'm looking at you Beaquin who died... but didn't, and Mersadie who was killed by that daemon... but didn't). Sometimes the women of the Ghosts read like Baywatch in Camo. Related to this, he has a bit of a thing for characters being very specifically copper-skinned, but that's more harmless recycling.
Abnett also has a big problem with endings. More often than not, his books will end with a large explosion and then a one page wrap-up. This is especially noticeable in his earlier Ghosts books, which trend towards the shorter side, culminating in a ridiculous number of plot threads in Sabbat Martyr being resolved in the span of about twenty pages. Also rears its ugly head in Legion, which is essentially a book worth of build-up to a single paragraph.
He wrote "I Am Slaughter" for the Beast Arises series. Which while a good book, did not have Orks until the last couple of chapters. And even then, they didn't do anything except show up. You can argue that their attack moon was the entire plot of the book, but who wants to read a book where the main antagonist is the gravitational pull of an artificial moon?
The verdict? That's up to you. Abnett's still a kickass writer, but whether or not he's always made the best choices in 40k is up to debate.