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).