The designer of these shelves considered the lateral forces that affect the construction of a typical bookcase. He or she concluded that the majority of a standard bookcase's structure was dedicated to resisting those lateral forces.
As an alternative to all that structural bulk, the maker decided to attach the back of the shelves to the wall and suspend their fronts from the ceiling via cable*, which solved the lateral forces conundrum. The prototype was made out of MDF and 3/16" metal cable and came to about $35 in materials. The shelves in the pictures, however, are made out of maple. To read more about the project, follow this link to 2 Modern.
*We assume the eye-hooks were screwed into studs.