The USB 2.0 specification is available free of charge from https://www.usb.org/document-library/usb-20-specification. On the right, you will see a link like usb_20_yyyymmdd.zip. Download and unpack the zip file, and the core specification can be found within as a file called usb_20.pdf (alongside a bunch of errata and additional specifications). Note that the date on the cover page is April 27, 2000 - and actually, the portions of the specification we are implementing are unchanged from the earlier USB 1.1 specification.