why give away the web version?
Maybe the free version cannibalizes sales, but I'm certain the book is much higher profile and reached a much larger number of potential customers because I gave (and continue to give) the web version away for free.
Especially with a niche like this, until you're an established author that people will just buy because it was written by you, people will require some sort of evidence that you know what you're talking about. Is the book worth it? Reviews are good. Better? being able to skim or read the book before buying.
Due to how temporary websites are - I think the physical copies and the ebooks are a sound purchase for people that will want to be refer to this book at some point in the future.