This is intended as a hidden doorway, but it's a reasonable way to design closet doors. That's because you're not expecting furniture to be placed in front a closet door. So, adding shelving makes use of that otherwise dead space.
Your standard bookcase-as-door secret room There’s added personality injected into a space when you’ve got a secret door or secret room. And it helps if that door doubles as a place to keep your books, too.