Black Library

The publishing company responsible for releasing fiction set in the Warhammer Fantasy and Warhammer 40,000 universes.

While the Black Library has published good books from good authors, some works are of dubious adherence to canon. Here's what Marc Gascoigne, former publisher and editor for the Black Library (and the dude who pretty much made it all happen) has to say on the matter.

...

"Keep in mind Warhammer and Warhammer 40,000 are worlds where half truths, lies, propaganda, politics, legends and myths exist. The absolute truth which is implied when you talk about "canonical background" will never be known because of this. Everything we know about these worlds is from the viewpoints of people in them which are as a result incomplete and even sometimes incorrect. The truth is mutable, debatable and lost as the victors write the history...

Here's our standard line: Yes it's all official, but remember that we're reporting back from a time where stories aren't always true, or at least 100% accurate. if it has the 40K logo on it, it exists in the 40K universe. Or it was a legend that may well have happened. Or a rumour that may or may not have any truth behind it.

Let's put it another way: anything with a 40K logo on it is as official as any Codex... and at least as crammed full of rumours, distorted legends and half-truths."

They have a regular submissions window so you can submit your fanfiction work and get a job a freelance writer. Like any business, they prefer to hire staff with an established track record, so the commisions tend to go to existing tie-in writers or ex-GW staff, but anyone can apply for the job. Fa/tg/uys have, as yet, been unsuccesful, leading to hilarious Nerdrage.

In Warhammer 40,000

In-universe, the Black Library is the repository of the Eldar Empire's collected knowledge of Chaos. It is guarded by the Eldar and hidden in the Webway, because that knowledge could be used for great heresy, although now and then, the Harlequins let someone check out a book if it is important that they know something. Ahriman has attempted to enter the Library for some time now; unfortunately, he has yet to succeed and stir some shit up.

Doesn't seem to be guarded that closely, since Inquisitor Jaq Draco could just walk in, steal the Eldar's book of Prophecies and walk away with it. The Harlequins let him in. He just didn't know it then boasted everywhere about it.