This is the Polonia Book Store in Jefferson Park. Polonia Books could have been so much more according to this article. Unfortunately, it won't be anything at least on the brick and mortal side. It does appear to still have an online bookstore presence.
I drove past it yesterday and saw that it was empty. For the last year or so, there was a sign above the door saying the space was for lease or sale. I guess the owner finally found a buyer.
Update: I should add that I always thought the space would be good for a Running Clothes Store, cybercafe and yoga studio. If I ever win the lottery I might hire one of my MBA peeps to write up a business model.
Update 1/19/2015: It appears that the bookstore has simply relocated to a much smaller space across the street and a little to the north, possibly sharing the space with another retailer.