Hidden doors -- This company makes cabinets, bookshelves and anything you can think of that fits your existing doorjambs thus hiding the door (and also making use of the space) Not sure I like the idea of a 'gun cabinet' door.
50 home library design ideas. This design is actually very easy to do. Use a room in your home that has a bedroom just off of it and install (or build) bookshelves on the wall were the bathroom door is. Change out the bathroom door for a bookshelf.