Here's my concern with ads. Even if I managed a 2% CTR with a subtle ad, I'd still have 98% of my users carefully avoiding a part of the page that could be devoted to something else.
Someone mentioned affiliate links in a bibliography section. I could see that working because it's not intrusive, but you'd have to be careful not letting your sources be dictated by what's available on Amazon. I like donations because it keeps the financial goals of the site inline with creating the best experience.
I could see sponsorship links working, maybe on the homepage, or maybe in a separate /sponsors page. I'm not completely sure, it's a tricky problem.
Someone mentioned affiliate links in a bibliography section. I could see that working because it's not intrusive, but you'd have to be careful not letting your sources be dictated by what's available on Amazon. I like donations because it keeps the financial goals of the site inline with creating the best experience.
I could see sponsorship links working, maybe on the homepage, or maybe in a separate /sponsors page. I'm not completely sure, it's a tricky problem.