Luckily, I found used copies (Lonely Planet Korea and Seoul; minor wear and tear; some writing on the pages) for sale online by browsing the buy/sell forums on Dave's ESL Cafe and WorknPlay. I would highly recommend browsing these sites before you buy a new copy. This will save you money (who doesn't like to save money?).
Although our copy of the Lonely Planet Korea was an old edition, the majority of the info (such as Korean translation) and maps (including subway) remained the same. If you would rather buy yourself a fresh, new copy then heading to Amazon is the way to go, while supporting The Daily Kimchi in the process.