Many writers are taking advantage of the new and wildly popular eBook market. Although I personally prefer the feel of a book in my hands - I have to admit, ebooks are convenient, portable, and less expensive than traditional books. Publishing an ebook is also a lot less costly and frustrating than traditional publishing.
Choose a topic, something you feel you can explain really well and in great detail. Make a list of what you would search for if looking for information on the subject and use Google's free keyword tool to see how many people are searching for your subject. If there is not enough quality information on the subject, and/or you've found a way to "build a better mousetrap" so to speak, an eBook may be a great idea for you.
You can self-publish and promote books on your own if you want to keep 100% of the profit and are very dedicated. Keep in mind sites like Amazon don't typically let individual authors list their books on their website. This is reserved for large publishers. For this reason you may want to consider self-publishing with a company like LuLu. This is not free, they take a cut, but they also put your book together for you (ebook, hard copy or both), take care of registering the ISBN (free), distribution, and much of the marketing. Lulu reaches a large potential audience and allows you to do both print and ebook formats for an affordable price.
If you are going for strictly selling electronic products companies like Payloadz.com can also be a cost effective option. This service has a free or paid option that allows your customers to securely download your ebooks instantly upon payment. It offers several payment options as well which is a plus. You can have your ebook hosted, promoted and have a great secure way to sell it.
The video below shows the nuts and bolts of how to create an ebook - converting a document to a pdf - very useful information.