I speak as a non-expert, but I'd have thought to make more than a pittance from advertising would take tens of thousands of visits to your site, and where are those visitors going to come from? Unlikely to be lured by the promise of a free book when there are so many to download on Amazon, unless it's blindingly attractive for some reason, in which case you'd be better off selling it. (In any case, how are you going to get so many people to even know your site exists?)
I've recently read a couple of books by self-pubbers who have popular blog-sites (largely because they blog about self-publishing) and both said that they make very little from advertising.
I think you'd be better off either offering one book free and others for sale, or offering a large free sample (say 50% of the book).