These IKEA bookcases are connected together, then mounted on casters and large gate hinges to create a secret door. A “how-to” article is located on IKEA Hackers.The extra wide door pivots open to reveal a hidden furnace closet room...
This secret bookcase door by Sunrise Custom Woodworking opens inward to a hidden room. The pull handle along the shelf gives some of the secret away. You can see this door has a shallow depth, and uses several standard door...
This secret door from HideADoor.com is made to look like a series of special purpose storage bins. The slanted shelves swing out to reveal whatever hidden room lies within.
A user on Reddit posted this secret door that was installed in their house when they purchased it. You push up on the clock, and the mirror swings in to reveal a secret room.