This is the most detailed explanation on howto explore a USB device I've seen. Need more articles like this!
I'm not saying there isn't useful information there, or that her book sucks. I'm just saying her website is a poorly organized mess that makes it less useful unless you know exactly what you're looking for. For a USB newbie it'd just be a maze of links, all alike.
http://code.rancidbacon.com/LearningAboutAtmelRZRAVEN
And if you happen to like reading about other people's (sometimes uncompleted) reversing projects like I do I've got a few more of those too: :)
http://code.rancidbacon.com/ProjectDigitalSpyCameraPen
http://code.rancidbacon.com/ProjectPyX86
http://code.rancidbacon.com/LearningAboutSkannerzRacerz
And a more general reverse engineering link collection: