I couldn't find this information posted on the site, so I thought I'd let anyone looking for English books know that there's a small library at ICANA (Maipú 672). I just went there today to ask about the library. In order to become a member you have to pay an 80 peso per year fee (or 45 for 6 months), bring a 4x4 photo so they can make you an identity card, and show them some sort of proof of residency (a bill or a contract). If you don't have proof of residency they told me that you can give them your address and they will send you a card and then you just have to bring the card in. 4 books can be checked out for 2 weeks at a time, and renewed twice. They have a pretty good selection of fiction and non-fiction as well as a lot of ESL teaching materials. They also have audiobooks and videos, but I didn't look at those so I don't know what the selection is like.
Hope this helps someone!
P.S.- ICANA also offers many standardized tests from the U.S. (GMAT, LSAT, GRE, etc.).
P.P.S- The Kel stand at the Feria de Libros had a great variety of English books, so I would hope that their stores would too.