Traditional Thai-Medicine is an ancient healing science. It is a cocktail of Indian Ayurvedic Practices, Traditional Chinese Medicine and genuine Thai folk healing and spiritual rituals.
Contrary to many Traditional Chinese Medicine or Ayurveda, it has never been formally systematized. The knowledge has been passed on from generation to generation orally, and written research is just showing up since efforts from various institutions in Thailand, specially the Ministry of Public Health.
There is a growing number of books on Traditional Thai-Medicine in English, and you can find most of them here:
Traditional Thai Medicine Books
Pierce Salguero's Books
There are some people outside of Thailand who also strongly promote and advertise traditional Thai-Medicine, Thai Massage and Herbal Medicine.
The most popular is definetly Pierce Salguero and his Tao Mountain. I have read Pierces books and they are well written, well researched and worthwhile reading. So I really can recommend you to go and get Pierce Salguero's books.
If you are into Traditional Thai-Massage, I really recommend you his Encyclopedia of Thai-Massage.
However, he wrote another nice book that on the herbal medicine of Thailand, it's full of recipies that you can use to enhance your own well being and background information you need and want to know. The book is called A Thai Herbal and it's well worth reading.
His third book deals with the religious, shamanic, spiritual side of medicine. Thai people believe strongly in spirits, ghosts and the supernatural. It's a fascinating, wonderful and mysterious world, and Pierce's Book The Spiritual Healing of Traditional Thailand can give you an insight into some aspects of it.