The Lonely Planet books are very useful for all budgets, covering backpackers to those who like to splash their cash. This book covers all the Thai islands and coastal areas, as well as an extensive section on Bangkok.
It includes details of transportation between all the different areas which is really handy if you are making your own way around Thailand, and there are tips on scams to avoid etc. It also includes the usual information of accommodation, eating options, things to do etc.
I would highly recommend getting this book if you are planning to travel around Thailand as it will be an invaluable source of information.