Also try Distral Books. I was looking for a book on the history of Buenos Aires and El Ateneo's computers kept saying they had over 100 of the books in stock, but (1) I couldn't find one on the shelf (Santa Fe & Callao location) and (2) The service, or lack-there-of, was downright comical. Absolutely NO ONE wanted to stop their conversation with their co-worker to see if they could find the book for me. TWO different times.
Then I went to Distral Books on Calle Florida and not only did the guy not have to look it up to see if they had it, he immediately walked from behind the counter (without having to be asked), got the book off the shelf and handed it to me, then went back behind the counter to ring it up in case I wanted to buy it then. Polar opposite to El Ateneo.