All of the books I have purchased from Amazon show up automatically when you have your Android Kindle app linked to your Amazon Kindle account. Other books can be transferred using either your /public network drive share and wifi connection or direct connect via the cable that comes with the Nexus 7 plugged into your computer's USB port.
After you plug in the cable, the N7 behaves like any other USB device and you can transfer files from the Desktop's Windows Explorer program or whatever equivalent Desktop OS application you have.