Yesterday, Sarah told me about a bookstore in Itaewon called What the Book with a huge selection of English titles, reasonable prices and ordering options. They even have a website where you can search titles. If it is not in stock a delivery time is listed. The book I was looking for can be delivered in 10 days. Also, I compared the prices on the book I was looking for between Amazon and What the Book and the WTB price was the Amazon list price. So while the Amazon price was less, you get free shipping with WTB so it evens out and you are not being overcharged. With Amazon, you can get free shipping as well, but overseas that can take a long time so the convenience factor comes into play as well.
I have the APO and Amazon is usually pretty fast, but it is nice that there is this option for those who don't have the APO. It is also work a look to see if your title is in the shop so you don't have to wait. She said they have a big selection of magazines as well.
Update: What the Book just moved to a new, bigger, more convenient location on the main street of Itaewon. Get off at Itaewon station. It is under Wang Thai. Click here for a map.