I've written two books I would consider a "success".
The first was a guide on how to write Facebook apps. I wrote and formatted the ~20 page guide in a single weekend. At one point it was bringing in about $100 a month. Unfortunately, the FB interface changes so fast that it became obsolete within a few months. I edited it several times, but I couldn't keep up.
The second was another ~20 page book on how to create and publish a book on Amazon using free software. This book was also written in a weekend and has made $20 to $30 per month since it was published 4 years ago. It generally gets 5 star reviews from users. I've never done much to market it. You'll see it at the URL below.
https://www.amazon.com/dp/B00A4112ZM
Even though I'm barely making any money on these books, it was still worth it. The latter represents about 15 hours of work and I've been paid about $1000. That's $65 an hour and it goes up the longer it's for sale and still relevant.
A couple of my things.
The book "Publishing for Kindle" that I published on Amazon for the Kindle and B&N for the Nook. It's a $2 book, so I dunno if there's any money in such things. I sold it from a website once too but I've let the page die because the few orders I've seen I got from Amazon and B&N. http://www.amazon.com/Publish-Kindle-write-ebook-software-ebook/dp/B00A4112ZM
A random guitar riff app I wrote for iOS. This one got me a couple free phones. Not much else. https://itunes.apple.com/us/app/random-riff/id797671897?mt=8
I built a system that would take your name and site name and automatically deploy a WP site for you. It's better than WordPress.org because you can install plugins and such, more like self hosted. Super easy to do, I imagine that could still sell well if I had some marketing chops. Site is gone, but I still have the software on my server.
I've done a lot of other stuff too, but after the domains expire and I haven't had any sales, I usually let the domains go and I try something else.
I've made 3 in the last couple months (https://splashofcode.com). I started making zines about 6 years ago when I wrote a tiny book on Facebook programming. At the time, I had never heard of a zine, but I wanted to share the info. I created it digitally and published it on Amazon for the Kindle. I created a few more. As books, many of them bombed (bad reviews). Then I published one explaining how I created them and suddenly people liked it (Publish on Kindle; How to write your ebook with free software https://www.amazon.com/dp/B00A4112ZM/ref=cm_sw_r_cp_apa_i_1pLwCbQ1C7N72). After that I came across The Sharpie Evolution. Suddenly my love for tiny books had a name and I was able to find others.