Bookcase door! I've got a small, maybe 8'x10' room that I'm going to make into my library. I'm going to attempt to do this to the door, depending on clearance, etc. Opposite wall will have a window seat with side walls having nothing but bookshelves.
I spent so many hours of my life during college in the library & still fondly recall using the card catalog. Would LOVE a vintage card catalog for my next home office. Already know exactly which items would fill it. :)