Magento. It's the leader in e-commerce platforms for so many reasons. And I can't find any platform that comes close to the features let alone the value. The large development community, the number of extensions available, and the significant documentation/education makes it a great choice for ecommerce development.
For a great overview of Magento, read Mastering Magento by Bret Williams. Whether you are a CIO considering a new ecommerce platform, the developer responsible to implement the platform, or somewhere in between, this is the book to help you understand the most about Magento. If you are already experienced with Magento you may find this book redundant in spots but unless you are really strong in Magento, you will likely find something to benefit you.