i would say, choose a theme just for that space and work around it! for example: maybe "asian" theme! paint the back wall RED or ORANGE, have a piece of buddha statue on the top most rack, you can use one shelve to just hold books with some decorative book ends, and instead of the chair, you can use the asian trunck boxes (if you google for asian trunks, you can see what i am talking about! - look for square boxes), this way, you can also tuck those boxes under the desk/shelf when not in use and the trunk could also be a storage for your files - you can also place a floor vase with some bamboo sticks - the asian theme would really work here as your wood is black/brown this would also turn out to be a peaceful writing space - enjoy!

Running the twin beds lengthwise against the walls, with headboards against the corners, usually will allow for the most floor space, but the trade off is that it is harder to make the beds that way.

A houzz contributor, not too long ago, had made a wine rack out of 4'x4' wooden pallets, that are used in the US to move a load of goods with a fork-lift truck. Perhaps they have them in your country. He cut them in half so they were only two feet deep and stacked them up to high counter height. The wood would be a nice contrast with your concrete, might add a little warmth to the space. cascio.offsite@gmail.com

How are the plans for the bedroom coming along?