UpNorth - Great question...
My humble opinion:
Soo Bahk Do Tang Soo Do Volumes 1 and 2 by Hwang Kee
These books read like a text book for our art. They explain every expect from how to move, when to move, and why we do things. There are also a huge resource of Hyung.
Tang Soo Do by Kang Uk Lee
This is my handbook for memory jogging. It has all of the basic forms and into some of the black belt forms. It is the most consice and easily transportable reference manual on our art in my opinion. I've gone through 3 copies over the years. I take this everywhere with me....you never know when you might need to look up a move from a hyung.
Living the Martial Way by Forrest E. Morgan
I believe any martial artist should read this. It appears frequently in any book thread on this site....so some others must share my opinion.
Small Circle Jujitsu by Wally Jay
This is not a TSD book, but it has advanced my understanding and ability in self defense techniques ten fold. After reading it and applying some of the basic techniques, I was able to execute our SD techniques much better and with a deeper understanding.
As a bonus, I strongly recommend having a copy of the Muye Dobo Tongji. You will learn nothing about the execution of Tang Soo Do from reading it. BUT, Hwang Kee based some of his work on it and it is definately a wealth of knowledge for Korean Martial Arts. I found it very interesting.