Benjawan Poomsan Becker's "Thai-English, English-Thai Dictionary" is clearly meant to be a companion piece to her excellent series of Thai textbooks ("Thai for Beginners", "Thai for Intermediate Learners", and "Thai for Advanced Readers"), and it will obviously be of most use to people who seriously trying to learn Thai. By having Thai in both Phonetic transcript as well as in the Thai alphabet, it means you will be able to use the book both before, and after, you have mastered the thai alphabet. The phonetic transcription system is the same as the one used in her textbooks, (and in other Thai-for-foreigners books published by Paiboon Publishing.)
As the book is aimed at people learning Thai, it does not assume the reader knows anything about the complexities of written Thai, but has a section carefully explaining the basics of the language, so that you can use the dictionary even without having any prior knowledge of Thai. (This section is really an ultra-condensed version of her "Thai for Beginners" textbook.)
Another extremely useful feature is the Thai alphabet guide, which shows examples of the many different font-styles that are used for printing the Thai alphabet. Believe me, even when you have learned to read the Thai alphabet, a different font may completely change the look of the 71 Thai letters, and have you wondering "Is this Thai?". The book is almost worth buying simply for this feature alone!
If you're just planning a trip to Thailand the book may be a bit of an overkill (unless you're linguistically inclined), and you might be better served by something like "Thai - A Rough guide Phrasebook" or something similar. Likewise if you can already read written Thai at a decent speed, this dictionary will probably be a little too light for you. But I would recommend it to anyone in between.