I wrote a whole post about swapping your bookcases for wall-mounted shelving. Wall-mounted shelving takes up less space, and can almost appear to blend in with the wall. Image from Stadshem, via Freshhome.
Its my dream to have a library ladder in my house someday. Floor to ceiling bookcases offer plenty of space to showcase a vast collection of books. A ladder gives access to books placed high on the bookcase.