This is the basic principle that was used by Hotmail to get established. When the two founders set up their free email system, all the messages that were sent by subscribers had a text message at the bottom which identified Hotmail as the origin. People who sent emails to their friends advertised the free email site.
Using viral E-books as a marketing method is cheap. It doesn't take long to set up and it's even quicker if you use rebrandable E-books that have been written by others. Just use your favorite search engine and do a web search. You will find many rebrandable E-books that are available on whatever subject you are interested in and that apply to your e-business. One method of distributing the E-book is to offer it to visitors in exchange for subscribing to your newsletter. If they pass it on to their friends and family it will promote your business for you.
E-books are capable of reaching a large audience and are limited only by the enthusiasm of the participants.
E-books are fairly easy to create. It's possible to produce your own E-books by combining articles that you have written or have gotten from public domain sources, such as directories.
A common approach is to use material that has Private Label Rights, including articles and reports that have been written specifically for that purpose, for that niche. Using a portion of a larger work that you have prepared such as the first three chapters of a large E-book could also be used as a viral E-book.
