It seems, at least using virtual reality, that the angle of rotation of the staircase between two stories is 90 degrees. If you design the library as a tessalation of space with repeating hexagonal pillars - as the sim implies when looking up and down the shafts, you will face a wall when reaching the top or bottom of a flight of stairs.
I would suggest an angle of 60 degrees rotation. This way, every cell on a floor can be reached from every othe cell by climbing and descending stairs for one flight.
With most other angles parallel rows on the same floor cannot be reached.
If 90 degrees is preferred, all planes must be rotated by 30 (90 or 270) degrees against eachother, changing the look of the shafts.
Did you know, that there exists one room that is filled with only 385 books in the complete library (number of books modulo 640). Perhaps the reception area?