Question for people who build stuff

Ok, I can't find this online so I don't know if it's possible or even how much it would cost so I'm hoping someone can tell me their thoughts.

We have a stairwell to a small rarely used closet, so these stairs aren't used a lot. I want to build a bookcase that is cut out like stairs on the bottom so that it can rest on the stairs themselves as it goes up each step. Think of like the stairs and the bookcase being 2 tetris pieces so they fit snug together.

Is this doable? Would it be expensive? Is it stupid?