Check out Galeria Krakowska. There is a huge bookstore on the first floor (unfortunately, I can't recall its name), and they have lots of books/magazines in English. I don't remember if they had international newspapers, but definitely worth a shot.
It's called the "American Bookstore" I think, located just near the food hall.
Last time I visited the one on the Rynek the only english magazine they could offer me was top gear for 40zl...
Empik has a lot of magazines in English.
For example, on the ground floor just near the cashier are all the current newspapers, as well as current affairs magazines (Time, Newsweek, The Economist, Poland Monthly etc.)
On the first floor, I personally head for the music magazine section - they stock lots of monthly magazines like Q, Uncut, Mojo etc plus NME (newspaper).
Other sections, I couldn't say if they have lots of publications in English.
There is also a large section with books in English on the first floor.
Don't get me started on the prices though. The Sunday Times newspaper for 60 PLN! I don't think so. I can just justify spending almost 40 PLN on a music magazine if you get a free CD, but it is still more than double what you would pay in the UK for the same thing.