Gyopo needs can be met in Gangnam by going to Kyobo Tower Bookstore and Dos Tacos around the corner.  The English-language section at Kyobo is sizable and extended browsing is no problem.  You’ll pay a little higher than retail for your new book, but the cost is no worse than ordering from Amazon with international shipping costs.  In passing, I spotted a neon “taco” sign on the big street and wandered in for a taste.  The hard shell beef taco was made with ground beef, lettuce, tomato, cheddar and jack cheese, sliced black olives, sour cream.  The store’s own hot sauce was a bit on the rough side, so use it sparingly.