The most important thing you can do to increase your odds of success when releasing a technical book is to have multiple pricing tiers, adding additional content so you give people the option to pay you more than $30 or whatever you are charging for just the book.

I released a book in May 2016 teaching PHP programmers how to refactor code with lots of loops and conditionals into collection pipelines.

So far it's made about $180,000 CAD.

~$30,000 of that came from the "just the book" tier, at $29-$39 (depending on if the customer bought at the discounted launch price)

~$75,000 came from a "book + screencasts" tier, at $59-$79

~$75,000 came from a book/screencasts/example application tier, at $135-$179

If I had only given people the option of the cheapest tier, I would've made less than half of what I made by adding additional tiers.

The absolute bulk of the value is all in the cheapest tier, in fact the screencasts are the same examples from the book + just two bonus examples, and the example application is a project I already had that happened to use a lot of the principles in the book.

You can take a look at the landing page here if you're interested: https://adamwathan.me/refactoring-to-collections/

I think this sort of success is pretty achievable, I just followed the strategy outlined by Nathan Barry in his "Authority" book: http://nathanbarry.com/authority/

Nobody is going to make a lot of money selling a $40 book through a traditional publisher. If you really want to produce content like this and be able to earn a living from it, you need to be a bit more strategic about it, which includes things like tiered pricing.

I don't think it's deceptive to call it a "book" just because the author put in the extra effort to give people the option to pay a premium price for some bonus material; that's the right way to self-publish a book if you actually want to earn a decent income from it.