It's a big spooky library full of dangerous knowledge, spiritual automata and ghost-fueled computers.

About the book:

The Stygian Library is a procedurally-generated library dungeon set in the extra-dimensional space between every library in the universe. Each expedition generates its route as it explores, resulting in new locations and forbidden knowledge being discovered with every visit.

Included within:

Systems for generating locations within the library, including Entrance Foyers, Map Galleries, Tea Rooms, Planetariums, Calculation Engines, Phantom Databanks, Paper Beehives, Phantom Pumps, Jarred Brains and the Sheol Computer.
40 monsters tailored to the Library, including Lantern Bearers, Skeleton Crew, Origami Golems, Guardian Shades, Lost Souls, Ink Elementals, Animate Spells, Archivist-Liches, Infernal Merchants, Escaped Fictions, Educated Rodents, Conceptual Wells and the orders of Librarians themselves.
A system for searching for specific knowledge within the library's depths, as well as 30 unusual books and the secret knowledge they contain.
Tables for generating treasure, dreams, rumors, and various other useful details.
A unique class of Mummified Sages, post-human academics who've dwelled in the Library for decades or more.

The adventure includes stats and sparse amounts of mechanics intended to be broadly compatible with most old-school RPG systems, and easily adapted to any other fantasy RPG.