All the bookstores and comic stores I found in madrid

Used English Bookstores:

Petra's International Bookshop
A small but cute second-hand shop, with books in several languages, very helpful girl was working there while I visited. She even directed me to the other international bookstore in the area when she couldn't find any books from Kurt Vonnegut, which I was looking for.

J & J Books & Coffee
Coffee shop upstairs, and a basement full of books in English. Very helpful people there, as well.

Comic Bookstores:

El Aventurero
I think this was the first one I found, right off of Plaza Mayor.

Arte 9
Doctor Esquerdo, 6
Cruz, 37
91 402 96 08

Elektra
San Bernardo, 20
91 521 39 75