I mostly focus on a short topic. For example, regular expressions, grep/sed/awk, etc. I prefer having plenty of examples and exercises while learning, so that's what I do while writing the books as well.

For producing the ebook, I use pandoc (https://learnbyexample.github.io/customizing-pandoc/) to convert from markdown to pdf/epub versions. I also use mdbook to create web versions.

I've also collected a few blog posts about book writing from other authors here: https://learnbyexample.github.io/my-book-writing-experience/...