Kiky, short answer is VERY! He is current President of Miyoshiike (sp) having handed the reigns over, so to speak, to his son.
No doubt at all that they are both very, very experienced in the art of this hobby of ours, for sure 'authoritive' enough to pay serious heed to.
By coincidence I just exchanged e-mails with the Rinko offices about purchasing this book, the conclusion being that I'll buy it at the All Japan Show next February. I look forward to reading it.