When you ask around among those who learned speaking Thai language quite well, there's only one group of textbooks that's mentioned again and again: The books from Benjawan Poomsan Becker (even the author of this website started learning Thai with them). If you don't know what to buy, try them out. Easy to understand, well-designed step by step approach, wide variety, and at quite low cost - Ms. Poomsan Becker did a good job.