Writing an eBook and selling it on your blog can be a great money maker. Your eBook should be directly relevant to your blog’s content so you can sell your book to your existing audience. Creating a recipe eBook for a food blog or an eBook full of training plans to complement your fitness site are just a couple of examples that have the potential to sell.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.