Used this book to travel in Thailand, Malaysia, Singapore, Vietnam and Bali (though I finally ended up buying another book for Bali as Indonesia itself is huge). Don't know if there is a better book that covers so many countries this well. Other people on tour had the Rough guide to SE Asia book and we found this one to be more useful because it had better maps and more information.
Obviously a bit tailored towards backpackers but you can easily find more upscale places (hotels, restaurants etc) in the "splurge" section.
Wouldn't dare to say that it covers everything but certainly a must-have for people traveling in the area