This documentation will give an overview of how the libusb API works and relates to USB. The documentation assumes that you have a good understanding of USB and how it works.
libusb is geared towards USB 1.1, however from the perspective of libusb, USB 2.0 won't be a significant change for libusb
The USB 1.1 specification can be found at http://www.usb.org/developers/data/usbspec.zip. The USB 2.0 specification can be found at http://www.usb.org/developers/data/usb_20.zip.