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.
While any “regular” job requires you to be at work to make money, affiliate marketing offers you the ability to make money while you sleep. By investing an initial amount of time into a campaign, you will see continuous returns on that time as consumers purchase the product over the following days and weeks. You receive money for your work long after you’ve finished it. Even when you’re not in front of your computer, your marketing skills will be earning you a steady flow of income.