28. Subscription – If you think of something valuable (newsletter, online magazine, etc.) that you can consistently offer on a certain basis (weekly, monthly, etc.), you may want to offer a subscription service. This could be a fee charged each time your product is sent out or on a monthly basis. Either way, this has to be something that your customers can only get by subscribing to your website.
You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.