This edition has plenty of information that is helpful. One can get enough information before going, upon arrival, or after arriving on where to explore without this book, however. The Lonely Planet Thailand guidebook (for the entire country and not just the islands and beaches) also has plenty of information on Thailand's islands and beaches.
With the inclusion of "luxury" accommodation and over-priced "tourist" restaurants and trinket shops, it appears this book is marketed towards a particular group: North Americans who only have two-weeks, and therefore need to "plan" ahead for their trip. This is a particular market and that is understandable, but it appears that LP is trying to get a piece of this market, and gear this guide-book towards these type of folks.
One thing to note: Ko Chang is not off the beaten track by any means. It's going to be the next place completely destroyed by the Tourist Authority of Thailand. They are working on it right now, evicting local Thais who've lived there for generations to build hotels.
There is no where in S.E. Asia that is "off the beaten" track, except perhaps, parts of Laos. The trail is crowded, and continues to get bigger. If you're a long-term visitor or a "two-week hoteller" then this book will due.