The "official" books are the ISTD technique books, as found in the Ballroomdancers.com book store. They are extremely informative, albeit somewhat dry. It is these books that people refer to when people are checking up on a technique or studying for an exam. They're great as study aids, but not the most exciting tool for learning new steps.
So depending on your situation, I may or may not recommend them to you. If you want to learn things for the first time, I would suggest videos. If you already have your repetoire down and you want to understand the technique better, then the books are great. Of course, a combination of both is good, too.
If you're really serious, the best solution is private lessons. Combine that with books and video, and you almost can't miss.
I hope this helps.
Sincerely,
Jonathan Atkinson