I released a book teaching beginner Django web app development: http://hellowebapp.com
Sure, it was quite a bit of work beforehand, but I've made around $25,000 from it so far, around $13,000 if you don't count the Kickstarter campaign.
The first book working so well is one of the reasons why I'm writing a followup book (which is fundraising on Kickstarter now: https://www.kickstarter.com/projects/1868398473/hello-web-ap...)
FYI: The best thing I did when releasing a book was to a have a super-tier with screencasts and videos. That one tier provides ~60% of the revenue from the sales of the book.