The Money Train is a great book being written by Benjamin Curtis about building e-commerce sites using Ruby on Rails. It's released as a 'beta book' meaning you can buy and read it in PDF form straight away, but over time you'll get tons of free updates and eventually the full, completed book (in PDF format). Even better, it's only $12!
It has some pretty cool 'e-commerce recipes' in it, so it's worth getting for those. It covers the basic such as products, variants of products, related products, and customer reviews.. through to gift certificates, shipping, carts, payment processing, and affiliate referrals. Over time it could turn into a true e-commerce bible, so for $12, it's worth having another excellent resource on hand. Thumbs up to Ben! [Ruby Inside]