Listmania!
So you want to learn Thai
By an Amazon.com customer
Thai-English English-Thai Dictionary for Non-Thai Speakers, Revised Edition (Dictionary)Thai-English English-Thai Dictionary for Non-Thai Speakers, Revised Edition (Dictionary) by Benjawan Poomsan Becker
Buy new: $10.20 / Used from: $9.24
Best dictionary you will find.  The phonetic system is difficult to use and not good for native English speakers.  Peace Corps might start using this one because the current one they have is useless.
Thai for BeginnersThai for Beginners by Benjawan Poomsan Becker
Buy new: $10.10 / Used from: $6.04
This one will give you a good start in the language.  Almost every volunteer ends up buying this.
ThaiThai by Pimsleur
Buy used from: $100.00
Because of the cost you can skip this one.  Too brief to be of any real use
Thai for Intermediate LearnersThai for Intermediate Learners by Benjawan Poomsan Becker
Buy new: $10.36 / Used from: $5.94
Good follow up from the beginner level. You can wait and buy it in Thailand and save a little money
Improving Your Thai PronunciationImproving Your Thai Pronunciation by Benjawan Poomsan Becker
Buy new: $11.25 / Used from: $7.49
Nice concept but falls flat.  It is pretty skimpy and not worth the money
Thai Reference GrammarThai Reference Grammar by James Higbie
Buy new: $25.74 / Used from: $25.30
Awesome book that explains just about everything.  If you buy it in Thailand it costs half as much