The Books of the Dead were created by the Deceivers, followers of the goddess Kina. The knowledge within them was granted to the Deceiver priests directly from Kina, and potentially possess the knowledge necessary to free her from her prison on the Plain of Glittering Stone, although they are never used.
After their creation, Rhadreynak, emperor of the land that is now Taglios and the Shadowlands, launched a crusade against the Deceivers, killing them wherever they could be found and driving their priests onto the Plain, both to protect themselves and preserve the Books of the Dead. The Books were stored in the ice caverns below the Fortress With No Name at the center of the Plain, where they remained for hundreds of years.
The Daughter of Night, through Kina, was able to read the books without physically going to the Fortress. She painstakingly worked to transcribed all three of them, but she was constantly on the run, so she was frequently interrupted and her books destroyed. The original Books are eventually burned by Sleepy when she reaches the fortress to free the Captured.