I spent a few weeks in Idaho over the summer and this book was my guide. Generally it was pretty useful, although a lot of the information it provided was out of date (guess that's the danger of buying a three year old guide book).
The local maps were generally accurate and useful, there is some good detailed information on more popular areas but if you're going off the beaten path you won't be needing to make space in your back pack.
I have a couple of other Idaho guide books and this proved to be the best of a bad bunch.
If you're just trying to get a feeling of what to do, where to go, etc then this will do you just fine, if you're a serious traveller then you're going to want to get additional maps and such.