This documentation will give an overview of how the v0.1 libusb API works and relates to USB. Work is rapidly progressing on a newer version of libusb, to be v1.0, which will be a redesigned API and is intended to obsolete v0.1. You may want to check the libusb website to see if it is stable and recommended.
This documentation assumes that you have a good understanding of USB and how it works. If you don't have a good understanding of USB, it is recommended you obtain the USB v1.1 and/or v2.0 specs and read them.
libusb is geared towards USB 1.1, however from the perspective of libusb, USB 2.0 won't be a significant change for libusb