My online free book search engine. Hope it helps.

I used to have a "Free books in the net" page that I didn't update much nor anything. It contained a list of technical books I liked, that were legally free to download from the net. People loved it, linked it, and published it on sites here and there. It was somewhat popular even if it was ugly, not updated often. I think if you do it well, you might have an interested audience.

A recommendation: look for other places where people look for books and try to interact with that people. Ie: configure a bot for the very popular irc Usenet #bookz channel, so that it serves only legal books and is 24/7 sending those books, but also advertising about your site. (First check with the channel owners if you can do that, but I've seen bots post url and messages on the chat room every x minutes).

It's just a Google custom search. You could make money with it if you SEO'd the hell out of it, probably, but if you're posting the link here you're probably not good enough at SEO to actually make serious money doing this. I don't think that this is a particularly profitable niche either. You want to find something where Google pays you $1 per click.

Keep on trying though. For this type of marketing discussion, I'd recommend digitalpoint.com. The most commonly said marketing idea here is "Make something people want and they will tell their friends about it."