Some E-books combine very well with an affiliate program. When the affiliate receives some extra benefit from promoting the viral message, like being able to brand your E-book with his affiliate links, the affiliate makes a sale when a reader buys through a branded link. You benefit from increased traffic on your website because the affiliate is willing to promote the E-books for you.
Be absolutely certain that your message includes a clear call to action. There shouldn't be any doubt whatsoever about what you intend recipients to do. Make sure that the instructions are perfectly clear so that recipients know HOW to act. The action that you require should be simple and obvious as well. Don't make things complicated or it will confuse the reader, such as by adding numerous links.
Remember the K.I.S.S. principle Keep It Simple Stupid! The incentive should be clear to those you want to take an action. Readers will look at your offering for about seven seconds before they move on. If it takes longer than that for them to figure out what it is you want them to do, they will move on. Don't make your message vague or difficult to comprehend. If you do, chances are you will lose them. You must respect your readers enough to express your message clearly.
Once you are sure you have an E-book that will be of value to an affiliate program, approach them with your idea and be able to show that it will be mutually beneficial.
Viral Marketing using E-books is one of many techniques that all together have a cumulative effect in attracting customers and subscribers to your business and in attracting targeted visitors to your website. You will very soon find out that this is an excellent way of increasing sales, subscribers and referrals.