When you are inside Jonggak Station head to exit 5 or 6.
It's pretty easy to find if you focus on looking for the signs that lead to exit 5 and 6. If you try to do what I used to (I don't know why, I just used to do it) and rely on your 'sense of direction' you may get turned around in the different corridors of the station. Just look for the exit signs with arrows and you'll find it.
Once inside you'll see a customer service counter--head to the left of that and walk to the far wall where the escalators are . . .
Immediately behind the cash desk is the section where English-Korean dictionaries and learning Korean language books can be found. I don't know why they have it there when the foreign books section is about 75 feet away--and we're the main customers for this section--but this is where they can be found.
When you get off the escalator turn left and walk around the escalator and towards the back right corner of the floor--this is where you'll find the English book section.
I've found some really good titles here for teaching methodology, teaching resource and lesson plan books, and also resource and lesson plan books specifically for teaching speaking and/or conversation lessons . . .
This is the teaching methodology section . . . it has a really good selection.
There are also the typical sections you'd find in any mainstream English bookstore--the prices are comparable to Bandi & Lunis . . . if you're looking for leisure reading you're probably better off going to What the book? in Itaewon, Seoul.