grodog on his site said:Library of Gresil. Three planes of row and rows of books, scrolls, tapestries, and other records enclosed in stone shelves. Paved flagstone floors and walls, ceiling that just go up . . . Gresil's library demons now who is permitted to delve the archives. This plane is neutral ground, at least temporarily, as few demons are willing to risk Gresil's wrath.
Are you familiar with the Library of Babel? It seems like the exact sort of thing Vecna would devise: a library of infinite secrets and obfuscation.
My old DM in college had a lord of demon lore, Gresil, and I appropriated him for my own games. His abyssal layers are a gigantic interconnected library stacks: