I'm currently writing an ebook about "surviving" martial arts. Basically it is going to be a guide on how to train for the right reasons and avoid classic martial arts pitfalls that cause students to get discouraged, injured, and fed up.
In the book I want to create a 'words of wisdom' section at the end. In that section I would love to have a bunch of snippets from martial artists of varying styles and experience levels on how they have mentally and physically maintained their training over long, sometimes challenging years.
I did an eBook (pdf) for my leananmean.com site devoted to fitness and weight loss. I too did not charge but it is kind of funny... I still stand by what I wrote but I've gone back to basics with dieting. I no longer thing low-fat eating is the only way to go but pretty much will go with any way of eating so long as you count calories in some fashion and increase your movement.
I think an eBook can be a great marketing tool - as you probably already know providing you are working towards increasing your traffic... and you yeah... you can do a valuable service to fellow martial artists too... you know - compatible strategies.