I think if you work hard enough to be that successful, by the time you are able to sit back and enjoy the money you cant make yourself do it as you have spent years doing nothing but work.

I would absolutely write a book for several reasons.

1. You become more successful by helping others succeed.
2. You have to do something productive with your time...Being wealthy and free is great but it can get very old soon, unless your involved in some type of productive action. Especially if he has 100 mill or 300 mill he has the ambition gene in him.
3. Making Money should never end.