A good website :
http://www.learningthai.com/
A good book, rather funny is "Thai for lovers". I saw many BG's learning English from it, but you can also learn Thai.
But the most difficult are tones. The same word can mean 5 different things if you pronounce it with the 5 different Thai tones.... so better to find a book with audio CD's ... and easier again to learn if you try to write in Thai.