I really like Niche Marketing on Crack by Andrew Hansen. It teaches a simple technique for targeting keywords and products with your affiliate sites. These sites are fast to set up, and you can start making money with very little traffic because the technique involves doing some very tricky but smart things.
I've posted information about how I create Niche Marketing on Crack sites and describe how these techniques work at my Badger Blog today:
Niche Marketing on Crack is Da Bomb
If you've been intimidated by the world of affiliate marketing, you'll probably enjoy his ebook because it teaches user-friendly techniques that will allow you to get started making money quickly.