I’m not sure how practical this is, but it’s extremely cool.  The Bookcase Door System from Woodfold offers a unique space-saving option from closets and pantries to safes and hidden rooms; can’t find a suitable place for your bookcase, just park it in front of the doorway! 
Woodfold’s Bookcase Doors are custom crafted to support up to 500 pounds, yet the heavy-duty roller system allows the bookcase to quietly open and close with a smooth easy motion.  You can tell a lot about a person from their bookcase!