I don’t think I’ve ever given a shout-out to What The Book?, a brick-and-mortar bookstore in Seoul that also delivers throughout Korea. Their selection is really impressive (they had this FFS) and it you’re looking for dead-tree publications they’re going to be your cheapest option outside of hit-and-miss used bookstores (in Daegu, that’s Buy The Book Cafe).
I’d say my reading these days is split pretty evenly between downloads for my iPad (on the subway, in a taxi, on an airplane, in bed) and good old paper and ink (I do a fair amount of reading on my exercise bike, where I don’t want to sweat on my precious iPad).
Anyhow, these guys are great and you should never suffer for a lack of reading material during your time in South Korea. Delivery is free if you order two or more items, and you can pay with a Korean bank card (have to enter your info in Korean), a foreign card via Paypal, or my favorite, doing a free money transfer via ATM from your Korean bank up to Seoul. (For some reason my USian bank has never played well with Paypal.)