This may be one of THE most powerful marketing tools in existence, especially since VERY FEW professionals or entrepreneurs are using this approach. DON'T try to sell a book in an ad. You don't have enough space. DON'T put all your material in your website. Or, if you do, take it all and also put it into a book.
People are much more likely to want a book than to visit a website.
And books will sit in their computer or bookshelf for future reference. Electronic booklets on the web are easy to create and distribute. If nothing else, you can always email people a Word file. The network marketing folks have perfected this art form. Speakers also use them as demo tapes.