I've tried a few books, and the Rosetta Stone software, with which to start studying Korean.
So far, this seems to be the best.
It is an updated version of an original book released about 10 years ago, incorporating much feedback from (American) students and teachers. The book isn't as dry as Yonsei University's offering. However, it is still quite technical, particularly the introduction, but is worth persevering with. Once you get to chapter 1, you'll find it is quite a user-friendly book. A particular benefit is that all of the audio files (5 for each chapter: 2 conversations, a set of vocabulary for each conversation, and a short paragraph) are downoadable free-of-charge from the publisher, which are great for listening to in the car.
I would have 2 suggestions for improvement. Firstly, it would be useful to have more audio files, particularly for pattern drills. Secondly, it uses the McCune-Reischauer system for representing the Korean alphabet in the Roman alphabet. The Korean governent did away with this in 2000, replacing it with the much more intuitive "Revised Romanisation of Korean". For some reason, however, academics persist with using the old-fashioned McCune-Reischauer system. Hence, if you go to Korea and expect to read road signs, you would be better off learning the newer system - or, even better still, learn the hangeul alphabet. In the book's next revision, I would want it to use the Revised Romanisation of Korean.
Once I have finished this book, I will buy the next in the series.
