I was thinking on similar lines yesterday and today made the decision of making all my ebooks free for the foreseeable future. I made bundles [0][1] so that they can all be downloaded in one shot. There are five books - three of them on regex (Ruby, Python, JavaScript) and two on cli tools (GNU grep and ripgrep, GNU sed).
Currently working on GNU awk, which will take another month if I want to include everything I had planned. Now, I'm thinking of releasing as drafts and see how it goes.
I plan to release book markdown source as well in coming days. Already done for Ruby [2]
[0] https://leanpub.com/b/regex