UPDATE: This store has apparently gone out of business?
This has at least a couple hundred English books, which are not the easiest to find. The bookstore is very cramped, but if you walk to the rear-right of the store, on the back wall there is a good selection of books in English.
The books are BEHIND moving shelves of Korean books, so you may not see them at first. Just push aside the moving shelves, and all will be reaveale!^^
Go out exit 8 of Hongdae Subway station, head up the hill towards the dongkyo dyeongsamgeori and the bookstore will be on your right, just before you hit the three-way intersection. It’s name is “book” 책: