Akb,
I don't know about anyone else, but the quality of instruction (where they studied, and under whom) and the personality of the instructor is key for me. Nothing puts me off like the one who boasted for almost three hours straight of his Thai massage prowess...seeming to forget that "do not boast of your knowledge" is one of the primary rules of Thai massage!
I've taken Thai a few different times now, and each time it adds to the depth of my practice. You can't ever know too much or think just one class is enough. This is something that truly takes a lifetime to master--and perhaps not even then.
One school I can heartily and without any reservation recommend is The Naga Center (http://www.NagaCenter.org). Not only was my understanding and body mechanics (something virtually ignored in previous classes) deepened to a whole new level, the instructor (director Nephyr Jacobsen) brought not only a most thorough knowledge of the subject but a compassionate, nurturing teaching style. Before you go all "ugh, touchy-feely", what I mean is, not only did she impart the necessary knowledge, but our class was evenly divided between those totally new to Thai and those experienced at some level. Body sizes and level of flexibility ran the gamut. A challenging task for any teacher.
Nephyr was able to teach all of us with ease and bring us all to a much higher plane of learning than we started with, without moving too quickly for the beginners or too slow for the more experienced. When it was important to be quiet, she was, allowing us to learn our own individual way. When it was important to speak up, she did. This is a very delicate art that even gifted teachers sometimes lack. She was also sensitive to the feelings of the persons who might not be comfortable with the morning and close of class chanting, taking pains to make clear the purpose and meaning of the prayer chants and the altar and inviting them to participate in whatever way put them at ease and was meaningful to their own personal beliefs. For me, this was the highlight. Nobody had ever before explained that the bodywork process is merely a clearing of energy so that the ending prayer can have the most effect. As important as the sen line work is, it is the prayer that is the whole purpose for the massage...how beautiful! This really changed my intent when I work and it is noticably more powerful.
As for books, please visit http://www.TaoMountain.org and get yourself a copy of Pierce Salguero's book, "Encyclopedia of Thai Massage". I have quite a few and this one is the most valuable by far. Descriptive and helpful as it is, you still shouldn't try it without proper instruction, as there is much that isn't explained (probably on purpose). But the rich background and history he provides is something you won't find anywhere else.
This should give you a great starting point. Sorry to be so windy, but traditional Thai is a healing artform I am passionate about.