There is a great English book store (if a bit expensive) with a great collection of serious books. It is in a galleria, right at the corner that runs from Santa Fe to Puereyedon (which I can never spell).
The lady there is very nice, but as I sad her prices are pretty high. I just paid A$R80 for a used copy of 100 years of solitude in paperback.
Good luck. And te guys at Walrus books tell me (and they import a lot) that English books are also duty free in Argentina.
Good luck.