The Sanctum Library is on the third floor of the building; the occult library is kept in a room structured in a pocket of folded space. If placed inside a mundane building Strange’s library would be twice the size the Sanctum.
The stairs, ladders and shelves of the library all move under their own power; the library shifts and folds to bring the patron and desired text together within.
Mystical Library Collection
Diary of Kenneth Ward
Marvel Database- Sanctum Sanctorum article- http://marvel.wikia.com/wiki/Sanctum_Sanctorum
Wikipedia Sanctum Sanctorum article- https://en.wikipedia.org/wiki/Sanctum_Sanctorum