Jon Emmet's book

I personally don't know the content of the book inside-out, but Jon has posted quite a few videos on YT with very useful info. Apart from that: I've met him last summer during the UK Open. His whole appearance and conversations with people attending that event showed that he has a purpose in life: making sure that people around him become better (Laser)sailors.

On those grounds, I would say that buying the book is a good idea.