I deal almost exclusively with ebooks.
The most important thing is to target a niche that isn't internet marketing. It's just oversaturated.
A good way to find out what ebooks might sell is to check out Amazon's top 100, and you'll see what type of books people are buying right now.
Once you write one, article marketing and forum marketing are great for sales. Also, send free review copies to blogs with high PR and they'll put a link to it on their blog along with a review, and you'll get -tons- of sales from their readers, plus great backlinks.