How to hide contents of a cluttered book case or kitchen shelves

 I would hang a pretty shower curtain over it. 

Or a curtain printed with what appears to be a bookcase or shelving containing lots of books.

Choose one which is easy to pull back when you need to reach the contents. 

Alternatively two curtains, with the option of tying them back at waist height.

Useful Websites

https://en.wikipedia.org/wiki/Marie_Kondo

The curtain shown above is this

https://www.amazon.com/QH-Bookshelf-Blackout-Insulated-Blocking/dp/B07V4HVQG7/ref=pd_lpo_201_img_1/136-8449709-6365034?_encoding=UTF8&pd_rd_i=B07V4HVQG7&pd_rd_r=251cb5a1-a605-469d-96aa-f37f3623dee5&pd_rd_w=sc0g4&pd_rd_wg=tZ3MB&pf_rd_p=fb1e266d-b690-4b4f-b71c-bd35e5395976&pf_rd_r=K0KKFYTWA6ZXJY6M9Q6P&psc=1&refRID=K0KKFYTWA6ZXJY6M9Q6P