This book contains three letters from a Zen master, Takuan Soho. You could read it in a day, but it has lessons to keep you thinking for a lifetime.
I've gotten through the first letter this afternoon and it went into so many things I've been pondering. Technique and Principle, "No Mind", Body/Mind/Spirit, and more.
I also picked up Sould Sword and Budo Secrets. None of these are technique books, but they do look to have a great influence on training if you are seeking something deeper than how to block and punch.
Ch'ang Hon Taekwon-do Hae Sul: Real Applications to the ITF Patterns Vol:1
by Stuart Paul Anslow
This is perhaps the best book on the martial application of kata that I have seen. The only other thing that even come close are some books on Yang Form Tai Chi but this is much better. It is the one TKD book that I have no reservations about recommending.