I thought I'd try some more very quick edits. The top-right bookcases have angled shadows.
I wanted to see if they read more like shadows to my eyes and they seem to. They can also be used to make the top bookcase look more like it's touching the wall. It seemed to me not to touch the wall before the edit.
You'd probably want to do some thinking about light sourcing and how the shadows would work if you wanted to go this route, it was just a proof of concept. It is opening up a bit of a can of worms though.
(There's a lot wrong with this edit, like the shadows on the right-hand wall, the shadow on the back wall being too narrow, some of the floor detail, etc. It's not meant to be a final product, more a test.)