I have bought books in Kyobo. There is one at Seomyeon and one in Centum City Sinsegae Department store that In know of. The Centum City store is very big and has books on Korea which would be suitable as xmas gifts.
I also highly recommend What the Book http://www.whatthebook.com/index.html they are a store in Itaewon, Seoul and this online store. Great selection usually.
There are dozens of book shops selling books in English about Korea. What sort of book do you want?
If you want a big heavy book full of beautiful, if somewhat cliched, pictures of Korea there are plenty available. Yeong Kwang (sp?) book store in Seomyeon used to have one of the best selections in town downstairs. Take exit 13 and walk straight, it's in front of you to your left.
The book I personally recommend if you want to give someone an interesting insight in to Korean history, politics and culture presented in a very informative and entertaining way is Rhie Won Bok's Korea Unmasked. It's a graphic novel, or cartoon if you will. He himself is a famed Korean political cartoonist.
Here is a review http://www.amazon.com/Unmasked-Search-Country-Society-Graphic/dp/8934917717 note that the cover has since changed. It is also available in most major bookstores.
Then there are a bunch of more academically orientated books on both the north and south. Bruce Cummings seems to be the biggest authority here. My father has read several of his books and says they are informative and fascinating.
Thank you both very much for your helpful replies. I will check it all out - I looked at Whatthebook but couldn't seem to find anything about Korea suitable as a keepsake gift.
Thanks again for replying.