Will it increase readership of my book if I rewrite it using the same software as Stacks Project book to make it available as interactive HTML on the Web in addition to currently available PDF?
Yes, in theory more channels = more ways to discover your work
Nobody can give you a definite answer to this question. You would have to simply try it to see whether it works or not. But it's very unlikely that this will increase your visibility. Judging by the question you posted on Academia you want the world to know about your work. Having another channel would of course offer a new way for people to discover it. But the thing is: the Stacks project is not that big compared to simply offering the PDF and LaTeX on GitHub. GitHub is already pretty big and most people familiar with your field of work will already know about it.
The best thing would be to simply put your work up on ArXiv, but according to your Academia post you are not allowed to submit there - which means that the general audience that might be interested in these things will not be likely to know about your work.
Offering multiple channels might increase the visibility, though it won't do much alone. You have to advertise your work somewhere where reputable mathematicians are likely to see your work. Mentioning it on Academia and here on Writing is already a good step in that regard.
What you could do is to start a Blog and informing people there about the implications of your work. Nobody will be interested in your work if there is no way to make it applicable for something - but if you devote a Blog to explaining how your research is relevant you might very well attract some readers, who might then tell their colleagues about your work.
Your time would better be spent on advertising the implications of your findings than on offering another channel for people to read your work.
A Blog would not be another channel, as it would be devoted to a related topic and not just a mere re-skinning of your already done work.
The points about whether the software is difficult to use and what the problems are with the platform you have chosen are irrelevant - you probably wouldn't have any problems and it might theoretically make a few people interested in your work, but without getting someone to look at the result this won't help you. If you've got the Blog or something similar going you could later offer another channel of the original work.