It's no wonder that I enjoyed this post ("37 Home Library Design Ideas") over at Freshome:
I have to say that if I were ever to draw up plans for my own dream library, it would have to contain a mezzanine. It's the greatest architectural feature known to man. Hands down.
And if said mezzanine were accessed via a secret doorway? Well, all the better:
Lots more here. Take a gander.