I'm constantly seeing more people coming out with books for free or just a passive donation link. This makes me immensely happy seeing as how they're leveraging the available free resources (Latex, CC-BY-SA content, free software for graphics) to make more resources available for free.
Open software is one thing, but a book is much more permanent in my opinion. A book like this will never go 'stale' or old like software does. We only need handful of good books for every topic out there at which point we can basically not buy books anymore. For many topics, I hardly have to consider buying a book since I can just use a free book offered by a professor. And watch course lectures.
What I want to say is this: Please write more for free. It doesn't matter if there is not much interest in what you are writing. It will help you too!