Not sure I'd lean solely on a BackTrack-specific book, myself, as there is a LOT that can / will be done that isn't solely BackTrack (yes, I know the course is Penetration Testing with BackTrack, but the concepts go beyond the tools in question.)
That said... one book or reference website... (harsh request, as it's such a broad set of topics and info) I could go various ways on this, all dependent on who the person was, that I'm 'mentoring' While they may be limited at pentesting, how technical are they, and what's their background in tech, otherwise.
One book - I might lean towards Thomas Wilhelm's book:http://www.amazon.com/Professional-Penetration-Testing-Creating-Learning/dp/1597494259/ref=sr_1_3?s=books&ie=UTF8&qid=1348849211&sr=1-3&keywords=pentest
My reasons are that, whether in the course, or elsewhere, you need to know the basics, how to setup a lab environment to mimic what you're attacking (or for practice.) While it's not all 'specific' to PWB, it covers a lot of the areas you need to be familiar with, and methodologies, for progressing in the field. I could really point to other books, as well, so I'm not singling Thomas's out. You could also look at:http://www.amazon.com/Metasploit-Penetr ... ds=pentest
or another of my favorites (thanks Jason Andress and Ryan Linn)http://www.amazon.com/Coding-Penetratio ... ds=pentest
I like the last one, because it gives you more perspective on doing things for yourself, WITHOUT pre-canned tools, after all, you can't really rely on those, much when you take your OSCP exam. Ultimately, if the person was technical enough, I think that it would get my nod as the single book.
As for one website, I think it'd be SecurityTube, if for no other reason than it's rounded videos and topics, that can be quickly referred to, in a pinch (although, again, I could point out others)