This DIY secret door is from Angie’s Roost. They took a corner china cabinet, beefed it up, and added casters to the bottom. Then they hinged it to the wall so it could act as a door for the small closet. It’s not a total secret, because you can see the wheels below, but it would only take a wooden trim piece across the front to conceal that area.
This particular design does take up a decent amount of floor space to open up, at least more than a normal door. But it’s relatively easy to duplicate, and the homeowners can make use of the corner which they wouldn’t get normally. They also get the novelty of being able to say they have a secret bookcase door in their home.