Hello,

First of all, a good abstract of the USB 2.0 especification can be found in www.beyondlogic.org (Device side)

Secondly,
If you want to develop Device's firmware perhaps this
may help you:

As I have said many times in this forum I am developing firmware for a W81E381AD microcontroller (8052+usb). As Winbond´s technical manual is really bad I have had to use other technical manuals from other manufacturers that have microcontrollers with a similar interface. The best manuals I have found are:
8X930Ax (Intel)
USS-820FD (Agere Systems)
(They even offer source code to achieve enumeration.)

I could also send you my code if needed.

I hope this helps you,

Xavier de Vicente