How well do free books work for promotion

I have several books published on Kindle, and use the free books avidly to promote them. I have had more success doing this than any other promotion strategy I've found. As an unknown author it is an uphill battle to get people to buy your book. So I had an idea this week to promote myself as an author by having a book that was always free. I tried to put this book on Amazon but found that the only option was to charge at least 99 cents for it. But I was able to put it on goodreads for free, so I did that. I wouldn't have been able to put it on goodreads were it not published, so I had to charge for it on amazon, and in order to give it away free on goodreads I had to give up the free kindle promotions. So I am not sure how all this is going to work, it is in an experiment. One thing I will say, this free book on goodreads is my favorite cover yet.