I don't know very much about Kindle but my guess is that when you say you downloaded two books, in fact what you did was download the first few pages and all the permissions needed to get more - so when you 'turn' pages, Kindle talks to the server and fetches the next few pages so they're ready. And because the app is talking to the server all the time about where you are, what's next to download, etc. it is using bandwidth.
Try installing a usage checker like Netbalancer. There is a free version, which doesn't have all the features, but it will probably give you a good idea of what are the main drains on your bandwidth. Netbalancer.com - go to Downloads and get the latest version. Hope it helps. Someone else may know of a better tool...
The Hubble Telescope has just picked up a sound from a fraction of a second before the Big Bang. The sound was "Uh oh".