Here are various resources that I check to understand what are the current trends in e-commerce and shopping.
A good general book on shopping is Shoptimism by Lee Eisenberg.
I also highly recommend a site that with almost exclusively e-commerce material is www.shop.org but you need to be a member to get the really good stuff. Great webinars.
And look at your Pew Internet & American Life Project research on the topic of shopping. Alot of good insights there too with specific reports on online shopping. Look in their "shopping" topic.