Ads are acceptable though it may slow down site growth. If the ads don't get in the way of your users, they won't mind as much. Also, a donation button might be worthwhile as well; but don't expect it to make much money for you. Consider for example the sob story at the top of the page: http://gnosis.cx/TPiP/. Two donations out of 1,000 book downloads a month.

If you can add some extra/useful features to the site, why not setup a premium subscription? Set the price low to start and that will likely get more revenue this way.

Good luck!