Most anyone is passionate and knowledgeable enough about a certain subject that they can sit down and write a thorough e-book about it. It can be anything under the sun. Choosing a very specific niche can lead to more sales of your book in a smaller-sized market whereas there will be more competition if you target a popular and lucrative market. Take a hard look at what you want to write about and decide where you want to fall on the spectrum.
Clickbank is a top marketplace for digital products. You upload your e-book there, create an advertisement for it, choose a price to sell it at, and a commission to pay affiliates who find buyers for you. You can just wait for their tens of thousands of affiliate marketers to promote it or you can also get a domain and create a related site/blog for it yourself. Then you can be generating sales of it from visitors to your website (and keep more of the profit for yourself). Depending on the subject you can create an ongoing stream of high-margin revenue for yourself with little to no maintenance. Ookabooka is a new e-book exchange as well.