A good affiliate ebook provides useful and current content in a timely manner. Ebooks vary in length and professionalism. Affiliate ebooks are sold in a variety of formats (although PDFs are perhaps the most common). Generally, when someone buys an ebook, they are emailed the PDF file or are directed to a dynamic URL (which will only exist for a certain number of days after payment). A similar system is to send a username and password that is only good for a few days. Make sure that your ebook reads well both online and offline. You can even embed hyper links, if appropriate, which are useful when people are reading your book online.