The OP might try the American Bookstore on the ground floor of the Architectural School (at Koszykowa 55, the western part of that street, 1 minute from pl. Konstytucji) which is bigger than the Arkadia one and according to the internet is still open.
According to a post on Facebook this week that bookshop has become a cafe. The Arkadia one is supposedly still open but may close in November.
Kindle seems to be killing book shops now, so go while you can.
What killed the American Bookstores in Warsaw was Amazon.co.uk delivering for free orders over £25. Now they have stopped doing that.
Although the American Bookstores in Warsaw didn't exactly help themselves: I remember going there once to try to order a book, even when I offered to pay when ordering instead of on collection they were not interested.