I write this because it amazes me how many people don't know about this place, or how many people underestimate it's awesome powers. If you live in Seoul and read in English--and especially if you're a teacher whose students read in English--What the Book is your lifeline.
What the Book is an oddly-named little bookstore in Itaewon, up the hill around the corner from the Starbucks and the Dunkin Donuts, on the same side of the street as the Foreign Food Mart. It's remarkable for being the only English-language bookstore in Seoul (at least, that I've found) that buys and sells used books. Their brick-and-mortar store is small, but cozy, and the selection of genre fiction is pretty good. You can also find a lot of titles popular among expats in new copies, and there's also a decent young-adult section. On the whole, a nice little store.
To tap into the full power of What the Book, however, you need to visit their website. What the Book operates a massive off-site warehouse, which they keep stocked with regular shipments from the US. The result is that you can order just about any book in print and have it shipped to your door in Seoul (or held at the store for pickup) for no more than the cost of ordering the book from Amazon and shipping it to an address in the States. If it's in the warehouse, you can expect to get it within a week; if they have to order it from the US, it can take anywhere from 4-8 weeks, but they always seem to have at least a few copies of the popular stuff on hand. Several times, What the Book has saved my life by getting us extra copies of class texts on short notice, and they're a great resource to share with students looking for independent reading.
In any case, What the Book is absolutely indispensible for English-language readers here in Seoul. (And you can use the visit as an excuse to eat some tasty Indian food at Taj Palace, across the street!)