This photo doesn't extend high enough, but my childhood years watching Scooby Doo has led me to conclude with 100% certainty the stairs lead to a secret spying room where you can peer through eye holes cut out of that painting.
After viewing all of these photos, I'm starting to come to the conclusion that the only reason anybody in this world keeps a collection of antique, leather-bound books is to hide a secret doorway. Man, my local library just got even cooler!
You know, I don't mind stepping inside and checking out your house's creepy, hidden, brick wall passageway. It would really make me more comfortable, though, if you didn't laugh maniacally and keep repeating, "After youuuuuuu!"
Look at the placement of the hinges on this door. It opens inward. That means, if you tried to go through and then close the door behind you, you'd end up leaving the bookshelf swung wide open. Conversely, if you try to come inside the other way, the shelf would block you and you'd be stuck outside.
It's the best kind of mystery passageway: one that makes no logical sense whatsoever.