Fundraise by recommending good books
You have a large supporter base of interested people visiting your website. If you recommend them a book, they will pay attention and possibly buy it. If they do, you will get a percentage from Amazon.
You can use this to recommend good books that inform your supporters about your cause. If you're campaigning against corporate takeover of schools, recommend No Logo. If you're campaigning against union-busting companies like Coca-Cola, recommend Mark Thomas' Belching out the devil.
This is an all-round win. Your supporters are reading books that they enjoy, and that inform them about your issue. They'll naturally be better educated and more passionate about your cause. And if you've got high traffic on your website, all the small percentages will add up to a decent amount that you can use to pay for more good work.
A big mistake would be to create a section of your website where you promote books. Nobody will look at it, and nobody will buy the books. Don't create a book ghetto. Instead, recommend books amongst your regular content. Intersperse your campaign messages with book reviews of good, relevant books. Before long, this will turn into a steady stream of income.