Bookcases with a ladder are obviously on the LustList, but why have a ladder when you can have a spiral staircase?
Karl Lagerfeld's library above stresses me out with its horizontally stacked books (doesn't it make your eye go crazy?), but still, that staircase is awesome.
This one, from Marie Claire Maison, actually just appears to be two different floors that both have bookshelves on the same wall, which has the visual effect of looking like a double-height bookcase joined by a staircase. Brilliant.
(Karl's library from Elle Decor)