Eli Bendersky's website This small booklet was recommented by a friend who used it while traveling in Thailand.
It's a really small book, about 130 pages pocket-format, but it's packed with information. A general overview of the country and its history is given, and the major tourism areas are listed, with many useful tips and suggestions of places to see.
The author also isn't shy admitting that the information may be incomplete and out-of-date and points to other useful resources of information.
The book has two other things that are very useful - a nice collection of Thai vocabulary, a few dozens of words / expressions that are good to know, and an alphabetic list of information with short and to-the-point advices.
I guess we'll rely on it as our guide to Thailand later this year (although we'll probably buy a map of Bangkok), so it will stand a real test. I'll report the results when we come back.