If you plan on being published in the next few years you should know that some e-book companies and publishers are preparing to add advertisements into eBooks. Imagine reading a book and an ad for soda pop comes up on the screen. These ads have both plusses and minuets.
On the plus side, these ads make your book available at a lower cost since merchandisers are purchasing the advertizing space in your book. You may even find a company to sponsor the book in exchange for advertizing space. They advertizing in your book in exchange you can provide your book to readers at a low cost or no cost and still get paid as an author. The more readers you get the more of a following you get, which is especially valuable if you are a career author.
If you so decide to do this be careful about what kind of ads appear in your book. For example if you have a Christian Romance book you don’t want your reader to “turn the page” and see an ad for pornography.
Of course most advertisers are smarter than that and usually try and promote to their target market audience. In the case of a Christian romance you may get advertisements for a Christian dating service.