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.
If you design a wildly popular app (which is harder than it sounds), you might be pleasantly surprised with the income they generate. Offering one or more apps at the iTunes or Android app store gives your creations wide exposure to prospects. And income can be generated by charging for the app, displaying in-app ads, or charging for in-app features and upgrades.
If you've ruled out survey taking as a legitimate way to earn money from home, listen to Bonnie Alcala. She and her daughter Andrea Spain, an elementary-school substitute teacher, take surveys online for fun and a little profit. They are careful to avoid any scams by refusing to pay an upfront fee or other charge. Bonnie and Andrea pocket around $100 a month for spending two hours a week taking surveys, which gives each of them extra pin money. In addition, they collect all kinds of gift cards and other prizes.
Some merchants run their own (in-house) affiliate programs using dedicated software, while others use third-party intermediaries to track traffic or sales that are referred from affiliates. There are two different types of affiliate management methods used by merchants: standalone software or hosted services, typically called affiliate networks. Payouts to affiliates or publishers can be made by the networks on behalf of the merchant, by the network, consolidated across all merchants where the publisher has a relationship with and earned commissions or directly by the merchant itself.
While these models have diminished in mature e-commerce and online advertising markets they are still prevalent in some more nascent industries. China is one example where Affiliate Marketing does not overtly resemble the same model in the West. With many affiliates being paid a flat "Cost Per Day" with some networks offering Cost Per Click or CPM.
You'll also need ecommerce software, fulfillment software, worry about warehousing, customer service, refunds and so on. But that's not all. You'll also need traffic. Think search engine optimization, Facebook ads, and other social media campaigns. Sound like a lot of work? Sure, it is. Especially if you do it all on your own. You could opt for Amazon's platform, which might be the easier route. But, then again, at the end of the day, this is a serious business, which could produce significant profits. So you're either all in or you're not.