London architect Tamir Addadi was "asked to convert the loft of a private house into a bedroom with a study, which was to include a large bookcase, whilst still keeping to a very tight budget."
But he didn't have a lot of room, so he turned the bookcase into a structural element that incorporated the stairs..
The bookcase vertical elements are notched for their full depth and the stair tread is slotted in and cantilevered out; there is no stringer on the opposite side.
I wonder if there is a bit of a bouncy diving board effect at the far end of the cantilever. But it is light, open, simple and effective. More pix at Mocoloco.