If you haven't read much about contacting spirits, you will enjoy this book. I think the book is credible because of all the stories Kelvin shares about his encounters with spirits. I attended one of his shows, too, last month, and he his very credible. He works hard to verify the spirits talking to him, and provides credible information to the person the spirit wishes to communicate with.
The book could have been edited a bit better. It is written in a very down-to-earth, folksy, prose with lots of New Zealand idioms, so a bit difficult for an American to follow at times. Nevertheless, it's a very interesting book and brings the author alive.