We created a hidden shelf in our office by simply adding some chipboard and wallpaper to a standard IKEA shelf.
We simply mounted a Billy bookcase on a Malm 3-drawer chest with L-brackets. The right side of this combination is screwed on wall, so it cannot move or fall in any direction.
This is it in real life
The outer side is disguised with chipboard and wallpaper, which you can easily hang on the chipboard. This way you don't piss off your landlord, the whole thing can be removed easily.
Here is a closeup of the chipboard with wallpaper mounted to the shelf
From the outside it looks like a real wall or chimney.
In addition you get a wall where you can hang some stuff.
When you enter the room you don't even notice it does not belong there.
On the top left you see the shelf (labeled "0,8 m²)
Also check out how we made this into a small Walk-In Closet in my guide "How to make a Shelf into a Walk-In Closet"