Either you are writing/planning to write a book, or you've already written the book. If in the first case, you decide to stop the project out of lack of prospect for financial reward, there wasn't any pressing impetus behind that book in the first place. If you have already written it, the effort that's gone into it is a sunk cost. Essentially I'm saying that the only books worth writing are the ones worth writing for free, in which case it's simply more noble and in the spirit of the thing to give it as a gift to everyone than to milk it for profit. The key thing here is that writing is very cheap in everything but time, is, thanks to the internet, non-zero-sum to consume.