I am a voracious reader , and have a huge collection home in the states. It got to be a big problem hauling books down here and searching for new , interesting reads. My son bought me a Barnes & Noble Nook for Christmas. I love it ! I really resisted , but now , I am sold .
Regarding Walrus , Nice chaps good selection. I took some books in to exchange , which they do , but felt I did not get such a bargain. It is like a walk back in time. You see books you read years ago. Plus , I live in Zona Norte. There are some stores with English book sections , and a full on English book store at the DOT mall , but again , selection leaves a little to be desired.
As for old books , there are lots of places with them. The antique market at Barrancas station on the train della costa is a good choice. Lots of English speaking ex pats out here in the burbs.