The Black Library is a location that is monumentally important to the Eldar. It contains their collective knowledge about Chaos, and thus is widely sought after by both those seeking to stop Chaos, and those who seek to be empowered by it. Hidden within the Eldar Webway, it is virtually impossible to reach for any outsider. The library is protected by the Eldar god Cegorach and his Harlequins. They rarely ever let outsiders into the library; one such example is Ephrael Stern, a Sister of Battle fused with anti-Daemon power. One person in particular, Ahzek Ahriman, has made it his life’s goal to find the Black Library and use it’s knowledge to become all-powerful.
The Black Library is also the name of Games Workshop’s publishing arm, with the vast majority of its novels published under that name.
TTS has merged the two concepts together by having the real world Black Library’s books appear in the fictional one. The Webway portal in the Imperial Palace leads directly to the Black Library, allowing Custodisi and Wamuudes to take said books and read them to the Emperor. This is the premise for the If The Emperor Had a Podcast show.