I think that Walrus is on Estados Unidos in San Telmo close to the South American Explorers clubhouse. But I don't know for sure.
I do know that there are a few used book stores near there (EEUU b/n Bolivar y Peru) that have a decent selection of English books.
But there is also the Feria de Libros that is on that island between the two sides of Santa Fe up in Palermo between the Plaza Italia and Palermo exits on the Line D subte.
Walrus (which has a nice selection) is in San Telmo, Estados Unidos 617.
The owners are quite nice and they buy back the books you buy when you have read them. It is not that far from Peru/Boliver/Catedral subte stops (they are all the same, more or less, on different lines).
My favorite book store is in an Argie style 'mall' They have a huge selection of English books. It is on the corner of Santa Fe and Pueyrredon. If you are on Santa Fe, you can just see it peeking out from the downstairs. It is on the same block as the Pueyrredon stop on the D line. The 'mall' has an antique shop out front, lots of 'Ferria Americanas' (Thrift stores), and a bunch of video game stores.