I used this book as a guide both when walking the Camino to Santiago, and when walking from Santiago to Finisterre this year. The great thing about the Camino is that the yellow arrows mean it is very difficult to lose your path but this book comes in handy in both giving you tourist information on where you're passing, but also in letting you know when to expect the next rest place.
The major disadvantage to the book is the distances - one pilgrim we met joked about the fact that gallego kilometres seem longer than normal ones, and it seems that the authors were also using their own measurements. The book claims that the walk is roughly 75km from Santiago to Finisterre - about 5km out of Santiago, we saw our first way marker which stated that there were 85km to go. The day from Negreira to Oliveiroa was a particular killer - what was claimed to be an undulating 26km in the book was actually a hard going 34km.
As a summary, this book is good for background information - but don't consider it too seriously as you'll find it very demotivating!