Not a book store per say...more of a "library"-type concept.
Not sure where you are located, but in Costambar...on the main road left hand side just down from the colmado.
You enter, what I believe, is the Costambar security building, (it is the first "office" in a series of ones, which include a pharmacie). Just walk through the building to the back room.
There are quite a few books, all done up in alphabetical order. The idea is to bring some of your own ones and leave them off in exchange for some of the ones there. They are mostly in English, but there is a small section of German books as well.
Hopefully this is still there...does anyone know?