Hello Carmine,
I completely understand. While a proper introduction to USB is certainly necessary, I feel that there is no need to re-invent the wheel i.e that information is available in other currently-available literature, so you may not need to re-introduce it in this work. That said, I’d like to humbly implore you to consider it, if you do have the time in 2017 or subsequently.
I think WereCatf’s suggestion is perfect: if you have free time at some point, then could you kindly consider doing a separate book specifically on advanced topics like USB and networking (using things like the LwIP stack for instance) ? That way, you can focus on finishing this one without any distractions, and can then (if time and space permit) work separately on the second book with relevant coverage.
Once again, I’d like to heartily commend you for a job well done, and I wish you all the best with this work and any subsequent ones.