I believe Amazon take a massive cut.
I have a friend who wrote a series of very successful U.K. cycling guide books and he said Amazon sales are barely profitable, albeit that they do have a wide reach.
His best sales are via publicity, review articles and general word of mouth within the cycling community, with purchases going through his own website.
It might be worth John setting up a website just to facilitate sales. Especially if other publications are in the long term pipeline. It can be a slow burn seller that brings in steady sales.
Looking forward to getting my copy.
Cheers,
Matt