Is he really considered an expert to you, solely because of the publishing of a book? Here it is by the way: https://www.amazon.com/Go-Programming-John-P-Baugh/dp/145363...
I know that's not necessarily your view, but this is the standard we are working with
I was on a meetup, where 4 scrum master "experts" were doing some presentation and they gave out their book. It was around 100 pages with very big font :) written by 4 people. So basically 4 bad essays of 25 page each.
But hey, they could then tell to HR that they (co)wrote a book on agile methodologies. Also they (co)hosted lectures for meetup attendees.
The more interesting part, is that this wasnt even the worst book about agile / scrum that I have read...