The author is exremely knowledgeable, a practical scholar and instructor, and a very good martial artist.
He has written a number of books for Turtle Press, mainly related to Korean Martial Arts (especially Tae Kwon Do). He has also written titles on basic training, flexibility, Martial Arts after 40 (one of the best on the subject, BTW), and he is the author who translated the Muye Dobo Tongji (Comprehensive Manual of Martial Arts of the Choseon Dynasty) into English.
Sang H. Kim has also done a number of instructional videos, covering everything from weapons techniques to kicking drills to strategy and technique for sports sparring. Some are better than others, but I have been able to use something from every book or video of his I've gotten.
I am not familiar with the specific book you've just purchased. Could you please put your own comments about it on this thread after you've had a chance to read it? Thanks!