Linux kernel documentation
[http://lxr.linux.no/#linux/Documentation/usb/:Documentation/usb]
[http://lwn.net/Kernel/LDD3/:Linux Device Drivers] - Read Chapter 13 on USB
USB intro
[http://www.beyondlogic.org/usbnutshell/usb1.shtml:USB in a nutshell] - A somewhat dated, but short overview of the USB 2.0 specification
[http://www.usb.org/developers/docs/:USB specifications] - official specifications from the USB-IF