The most famous guidebook for Korea, and books for the world, are the Lonely Planet series. I came to Korea with one and also bought a LP Seoul guide and a LP Korean Phrasebook. They were quite useful although I never liked the maps.
My roommate first shared his Moon handbook with me and I quickly bought one. The LP book was much more portable, I could throw it in even when going on a hike. The Moon guide was twice as long, a much bulkier proposition. We ended up photocopying the relevant pages before traveling.
Two weeks ago I bought the third edition, (cover pictured below).
After five+ years in Korea, why buy a guidebook? Well, I still haven't learned much Korean so I appreciate having the information in English. There's more than that: I've gotten old, I've settled a little. I still want to travel, but I haven't done much in the past few years. There are whole provinces still to explore and even large swathes of Gangwondo and Kyoungsangnamdo that I have not visited.
There are the usual disappointments in opening a travel guide. I remember seeing a guide to Canada, looking up my home district and reading that it was a pretty but overpriced, and overhyped destination with poor public transit. All true, but not what I wanted to see.
Another disappointment, with a Korean example this time, is the over-flowery language. Moon's Handbook, 2nd and 3rd editions both describe when to visit Chisokmyo in Kanghwa-do (a dolmen or combination stone gravestone/altar). "On an ordinary day it's a pleasant trip but a visit to this ancient relic might be more evocative on an autumn morning, when the crisp air tingles the nose and the fog is so thick that you must push through it, summoning visions of myth, mystery and wonder."(P255, 2nd Edition. P275, 3rd Edition)
I have to agree that thick fog may help. It is remarkable, when you get there, that those stones were set in their positions thousands of years ago but the location has no ambiance. The highway is right there, a fence surrounds the dolmen and there are only a few trees huddled to one end of the cleared area. There are other, smaller, dolmen located in the forest (somewhere, I have only read about them) that would be more shiver inspiring.
Still, I have enjoyed my Moon Handbooks and have already found the 3rd edition useful. It has already chastised me a little. In a previous post, I complained about farmers drying their rice across the whole bike path. The new Handbook has a two page spread about cycling that includes these words: Locals sometimes spread crops out to dry on the trail's asphalt surface; kindly ride around the harvest. Remember, it was their taxes that built the trail, and they're the ones that will pick you up in a minute if you need help. (P118) I'm a taxpayer in Korea too but I see the point.
The new Handbook is about 50 pages longer but the pages are thinner, so it's a more compact book. I will refer to it at home before trips and possibly make copies of relevant pages so I don't know or care if the thinner pages are more delicate.
It has a different opening. There is no title page and publishing information is all on the last page. The new Handbook starts with a two page color map then another two page map with relevant page numbers shown, very easy for navigation.
All the maps are much larger and clearer.
One of the big advantages the Lonely Planet series has is it's great website -linked above. Travelers can post questions and updates easily. For the Moon guides, I guess we just have to wait 6 years for the next one.