I made quite a nice amount of money selling pubic domain and my own e-books on Amazon and Barnes and Noble when the market just began to boom in 2011. For 5 years I cleaned up making more than 6 figures. However, then the bottom fell out, as it does in all gold mines. Why? Literally, millions of other prospectors flooded the market with good but mostly bad, spam e-books. This forced the big players to tighten their requirements, ban most public domain material and really go over all e-books summitted with a fine-tooth comb.
Plus, big publishers decided to stop boycotting e-book publishing and opened their catalogs to Amazon and Barnes and Noble. This killed the market for me and many others. However, it was great while it lasted.
* The motto of this story: get in on the ground floor of a new trend and you can make some big money.
Today, the e-book market is a waste of time for me.
For example, I spent 4 months writing a very good non-fiction e-book that is rated with 5 stars, only to barely make $220 in royalties in the last 8 months. Not a good investment for my hard work. I did this to test the waters after walking away from the industry a few years ago. There is little market for unknown writers, no matter how good -- in my opinion. I stopped writing and instead I spend my time looking for new trends. Good luck!