If you are like me and avoid tour guides as much as possible, but still like to know about the places you are visiting, then this book is for you. It gives excellent descriptions of the temples. It has lots of background information on the history and on the art you'll see. It also contains wonderful photography. Plus, it has lots of general information on traveling in Cambodia including a section on Phnom Penh. If you are doing a basic Cambodia visit of Siem Reap and Phnom Penh, then this book is all you really need.
My only complaint is that it is a big book, bigger than other tour books like Lonely Planet or Rough Guide. This is a problem if you are trying to travel as light as possible. However, I still recommend taking it.
A general suggestion for visiting the temples: be aware that they are now apparently the second most visited tourist destination in Asia. Lots of Asian tour groups everywhere. To avoid them, go early! Get going at 6AM (or even earlier and go see the sunrise over Angkor Wat!). Then you'll have the temples to yourself for a couple of hours. Midday, when the tours go back to their hotels for lunch is also good, as is late afternoon/early evening. But even with the tour groups around, it is still an amazing place!