Amazon's affiliate program is nice because you can throw in a link to Amazon in almost any article, and readers will find it useful/relevant. I'm always surprised when I see big sites linking to Amazon, but not including a referral code. They're throwing away thousands of dollars in potential income.
I had links to books on Amazon before I became an Associate because I discuss sources and references in many of my posts. Being an Associate hasn't made me any money yet though; too few readers, and I've been posting too infrequently to get more.
