One of my favourite stores is Aladin in Gangnam because it offers books and DVDs at magical prices! Although the items are second hand, they are in really good condition and often priced at 2700 won or so. Most of the books are in Korean, although you may find some English titles, and you can also check out their selection of Korean films (mostly with subtitles).
Aladin is located at the CGV building in the middle of the Gangnam shopping belt (halfway between Gangnam and Shin-Nonhyeon station). There are also other branches in Seoul and other cities in Korea.
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