I have the 2nd, 3rd and this 4th edition in the Moon series of Robert Nilsen's books on South Korea. I also have the Lonely Planet editions on Korea. When I go to Korea, and I have gone more than a half dozen times, I find that the only book I need to take is Nilsen's. It's hefty for a paperback because it's more thorough.
You get not only the expected tourist information on transportation, accommodations, places to eat, etc., but also a very thorough-going choice of locations and things to see and events to attend throughout the provinces and cities of South Korea, arranged clockwise around the map of South Korea, beginning in Seoul. The items Nilsen chooses range from entertaining to seriously historical and/or cultural. This book is for the traveler who wants more than a surface experience of Korea. In fact, Nilsen includes capsule cultural, historical, geographical, economic, language, etc. information and useful appendices, including a bibliography of more in-depth works on various aspects of Korea, all located in the back of this latest edition.
Nilsen spends 95 pages on Seoul [compared to Lonely Planet's 62] and is similarly thorough for the rest of South Korea, yet also includes, toward the beginning, a guide to seeing much of Korea's highlights for the traveler who does not have a great deal of time, whether only a few days or up to three weeks.
This is also an excellent book for the armchair traveler who wants to become acquainted with Korea in more depth than one can usually expect from a travel book.
I would suggest getting this book well ahead of a trip to Korea, so that you can familiarize yourself with the riches of this tome and therefore make your trip more meaningful, as well as more enjoyable.