The book is written by the people who run www.geocaching.com and it is to say the least, complete. It covers what geocaching is (sport, hobby, excuse to go outdoors and play with electronics), what you need to have, what you need to bring, and techniques for geocaching.
Geocaching is sort of a treasure hunt, orienteering, hike in the woods thing. The book takes you through the whole thing.
The section on how to use a compass along with your GPSr is worth the price of the book by itself.