What are the percentage of people that would make the the first chapter in and fail to convert - vs the people that would have bought then likewise never finished?
Because I have the entire book online, people are much more likely to reference and point other people to it because it's a complete useful web resource. If, say, someone on Reddit asks about learning some architectural pattern, it's natural for someone to say "Oh, check out this book here." It's less likely they would say, "Oh, check out this page which just describes a book that you have to buy to actually answer your question."
So my book is much higher profile than it would otherwise be. A lot more people are discovering my book and coming into the funnel. The fact that they can read the whole thing without paying probably means a smaller fraction of people buy it, but the increase in the total number outweighs that.
Also, giving it away for free gives people a lot of gratitude towards me and that in turn can cause some to buy it. It feels less transactional to them because they don't have to. In fact, I've had a lot of people email me saying they had already read the entire book online but bought a print version just to show thanks.